Wednesday, May 31, 2017

Re: [MW:26565] ISMB 400

Please refer to the Table QW/QB-422 of the ASME Sec IX-2015, IS 2062 (Plate, bars & shapes) are allotted P no. 1 & Group no. 1.

Regards


On Monday, 29 May 2017 17:37:42 UTC+5:30, George Dilintas wrote:
The allocation of a P number to material is done by ASME and by nobody else. If you allocate a P number on the basis of chemical composition them you violate ASME Code



Στάλθηκε από το smartphone Samsung Galaxy.

-------- Αρχικό μήνυμα --------
Από: Lakshman Kumar B <lakshma...@gmail.com>
Ημερομηνία: 29/5/17 08:40 (GMT+03:00)
Θέμα: RE: [SOCIAL NETWORK] [MW:26546] ISMB 400

Hi,
IS 2062 is Indian standard and we are looking the reference in ASME, and so we can't able to see the correlation at anywhere.

Yes – with the help of chemical composition correlation, we can group it to P1.

Further if the WPS is only limited for ISMB of IS 2062, then you can specify directly IS 2062 – no need to mention P number.

P Number is generally used to avoid multiple WPS which falls under the same P numbers & Group numbers

Hope it's clear to you…

Thanks & Regards,
Lakshman Kumar B,
+91 9440031459.

From: material...@googlegroups.com [mailto:material...@googlegroups.com] On Behalf Of Hemant W Joshi
Sent: Monday, May 29, 2017 9:50 AM
To: material...@googlegroups.com
Subject: RE: [SOCIAL NETWORK] [MW:26543] ISMB 400

Ifg i am not wrong IS 2062 is not covered under ASME. You need to check nthe chem.. composition

From: material...@googlegroups.com [mailto:mater...@googlegroups.com] On Behalf Of Amol Dhumal
Sent: Monday, May 29, 2017 7:55 AM
To: Materials & Welding
Subject: [SOCIAL NETWORK] [MW:26539] ISMB 400

I have to prepare WPS for IS 2062 E250 BR # ISMB beam, Please let me know what will be the P no and group no of this material ?
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[MW:26565] Re: ASME B31.3 -2014 Table 331.1.1 Soaking time for P1 material

For P no. 1 & 12 mm, the minimum soaking time for PWHT is 29 minutes & same is calculated from one hour per inch.

Also please note, the minimum 15 minutes in code, is for only if your value for minimum soaking time(after calculating from one hour per inch for respective thickness) is less than 15 minutes, for eg if you get 14 minutes then you should consider minimum 15 minutes.


Regards


On Monday, 29 May 2017 15:33:46 UTC+5:30, SWI wrote:
Hi Team,

Could any please clarify
What is the minimum required soaking time for P1 material with wall thick 12mm, as per ASME B31.3 year 2014 Table 331.1.1 .

Is it still Minimum one hour per 1 Inch wall thickness or Minimum 15 Minutes...


Regards,

Ram

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Ensuring Safe Operation Of Vessels With Quick-Opening Closures

Ensuring Safe Operation Of Vessels With Quick-Opening Closures 
D.C. Perreira 
Category: Incidents
 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN.


Pressure vessels with quick-opening doors command a great deal of respect from the Boiler Inspection and Insurance Company of Canada (B.I.& I.). We know by simple calculation that a pressure vessel eight feet in diameter with working pressure 150 psi, has an end force of just over one million pounds acting on the heads. This does not worry us unduly in a conventional pressure vessel with welded heads. However, when the head serves as a door and it needs to be held in position by some form of locking mechanism, we have to be more concerned. Brick curing autoclaves, metal bonding autoclaves, autoclaves used in the textile industry, wood treating cylinders, tire vulcanizers, even sterilizers can all be likened to a loaded rifle with the hammer cocked when in operation. It doesn't take much to set it off. It's a lethal weapon and it needs to be handled very carefully.

Our respect for pressure vessels with quick-opening doors does not stem from theoretical calculations alone but from bitter experience.

Some 12 years ago, an autoclave failure occurred at an insured concrete plant in Hamilton, Ontario. An 8'6" diameter by 108' long brick hardening cylinder, one of six installed in the plant, had the door blown off just as the steam pressure reached its normal maximum 145 psig at the start of a curing cycle. After deflecting off the low wall of the loading bridge pit, the door of the autoclave pierced another wall and continued piercing the plant research laboratory located over the steam kilns. The walls and a large portion of the plant's roof collapsed from the pressure of the explosion.

The 45 ton autoclave moved 150 feet away from its foundation and destroyed a delivery truck, curing racks, and numerous cubes of inventory block. Fortunately and miraculously no one was seriously injured.

The incident was fully investigated and the cause of the door failure was attributed to reduced bearing area of worn wedges and possible slippage of the locking ring.

In June 1969, another concrete block manufacturing plane in Hamilton was leveled by an autoclave door failure. The B.I. & I. was not involved in this accident but we had previously insured the location and were naturally shaken by the disaster.

The newspaper report on the incident gives the size of the autoclave as 12 feet in diameter by 80 feet long and indications are that the door blew off during operation. The roof of the plane was lifted 10 feet into the air, one wall was knocked down, and steel plates and concrete blocks were hurled 150 feet into the air. The vessel tore free of its concrete supports and ripped across an empty lot into a children's club (thankfully the club was empty because the children were away swimming). The club was flattened and the autoclave continued for another 200 feet squashing three pick-up trucks and two cars before slamming into a wall of an autobody shop. The wall fell on top of at least four parked cars and the autoclave finally butted a Volkswagen 80 feet into the middle of a nearby street.

Unfortunately, one operator died instantly and three others were reported to be critically injured.

We do not have any details as to the cause of the accident but photographs 3 and 4 attest to the devastation. Note in both cases the plants appear to have been bombed. They probably were, if you accept the fact that the energy stored in autoclaves of this size is approximately equivalent to about 80 pounds of TNT.

Following these disasters, a special Ad Hoc Committee on Autoclave Safety was set up by the National Concrete Producer's Association. C. A. Williams, B. I. & I.'s vice president of engineering, represented the insurance industry on this committee that was responsible for the 1970 publication of the pamphlet entitled "Safety Precautions for Autoclaves." The foreword of this booklet is of special interest.

"In the past 20 years (1950-1970) about 200 concrete block manufacturers in the United States and Canada have installed autoclaves to high-pressure steam cure their products. Prior to 1950 there were about 20 autoclave plants in operation. In these 220 plants employing this curing process there are probably well over 500 autoclaves in use today.

In the history of the concrete block industry, there have been eight autoclave failures; one each in New Mexico, Texas, North Carolina, Virginia, Illinois, Quebec and two in Ontario. Seven of the eight were quick-opening door failures, and six of these seven were wedge-lock type doors.

A recent tour of autoclave plants in Canada pointed out major deficiencies that effected the safe operation in three of the twenty-one plants inspected. Most common deficiencies found were related to the autoclave door safety locking device.

Due to wear and tear and to a degree of over-confidence often associated with long term trouble-free operation, most of the autoclave doors should be thoroughly inspected NOW! Subsequent inspections by a representative of the door manufacturer or other qualified person should take place every three years. Weekly, monthly, and yearly inspections are the responsibility of plant personnel and insurance company inspectors."

A copy of this pamphlet was issued to each inspector in the B. I. & I. with instruction to read and study the material and retain for reference. The need for adequate overlap on wedges was stressed to all inspectors.

We do not know of any major disasters in Canada since 1970 but we have, from time to time, heard of incidents involving autoclaves that serve as grim reminders of the hazards involved in operating these vessels.

On December 3, 1974, an operator was killed at an aircraft plant in Brampton, Ontario, when the door of a metal bonding autoclave (an 8' by 30' vessel) was blown open by residual pressure as the operator was about to open it. The operator was slammed against a concrete wall and died instantly.

Newspaper reports on the inquest into the death of the operator indicated that the vessel was being operated without a safety device that had been recommended by the insurance carrier.

In April 1976, a 17-year-old youth was killed in a cindercrete block manufacturing plant in Regina. This incident involved a 10-1/2' diameter by 124' long autoclave with a Ring-Lok Quick Opening Door. The autoclave was being vented at the end of a cycle and pressure was down to 5 psig on the gauge when the young man, impatient at the delay in waiting for the pressure to dissipate, decided to open the door despite shouted warnings from a senior operator. The door flew open, striking the concrete wall and killing the youth.

On February 2, 1977, the door of a tire curing autoclave 12' 8" diameter by 9' long blew open at a plant in Montmagny, Quebec, shortly after it was placed in service. It completely demolished a 50' by 200' building in which it was located. Fortunately no one was injured.

In May 1977, a vessel used for curing retreaded tires blew open at a location in Toronto. This vessel bears little resemblance to the vessels mentioned previously except for the fact that the cover is held in place by a concealed locking device and is therefore analogous to the loaded rifle. No one was hurt and the vessel, which was badly damaged, was replaced by the manufacturer at their own cost. At about the same time a similar failure occurred in Alberta, and again no one was injured but building damage was substantial.

In August 1977, at a plant in Edmonton, the chief engineer and production superintendent entered an 8'6" diameter by 140' long brick hardening autoclave to examine the rails which carry the loading cars. While they were inside, the operator, unaware of their presence, proceeded to push the tram of loaded cars into the autoclave. The two men managed to climb on top of the stacked blocks and began crawling desperately along the 14 inch space above the blocks toward the front, but were carried inside by the train so they were 80 feet from the door when the train stopped. They got to within 20 feet before the operator closed the door, and their shouts could not be heard above the din of the forklift truck that was used to push the train into the vessel. As steam was being admitted, the men managed to remove some blocks and pull free a steel tray that they used to pound on the door.

A welder working nearby heard faint thumping sounds and mentioned it to the operator. But after both men listened near the door they could hear nothing unusual. However, they decided to shut down the unit and open the door anyway. Both men were unconscious but quickly revived. They had only suffered numerous cuts and bruises in their desperate scramble over the blocks.

Why do these vessels fail? In almost all cases it is a combination of improper operation and poor maintenance.

How can we ensure safe operation of these vessels? The answer is fairly obvious.

1.  The vessel must be designed, fabricated, and inspected in accordance with Section VIII of the ASME code.

2.  The vessel must be properly installed on adequate foundations with provisions for thermal expansion.

3.  The vessel must be properly operated. This is the responsibility of the owners or managers. Their attitude is all important. They are the ones who should ensure that operators are fully trained and re-trained for the job.

4.  The vessel must be properly maintained;

5.  The vessel should have the required complement of safety devices.

The human element cannot be overlooked in the prevention of accidents. All too often after a long association with an operation, even key operating personnel have a tendency to become careless or lax. Personnel without complete knowledge of the hazards involved in the operation are even more likely to become careless and fail to follow safe operating procedures. Thorough training of operating personnel is most important and special attention must be given to the following:

·         A complete explanation of the entire process and hazards involved.

·         A thorough understanding of routine duties and emergency procedures.

·         Thorough instruction and explanation of the functions and operation of control devices.

·         An explanation of how safety devices function.

·         Proper procedures for depressurizing the autoclave before opening the door.

·         Proper procedures for operating the quick-opening door.

·         The dangers involved by forcing operating mechanisms.

·         Instructions to report at once any malfunctioning of equipment or control devices.

·         Instructions should include where to look for wear and evidence of possible failure of equipment.

It is fairly certain that the autoclave disaster in Hamilton stemmed from the fitting of an oversized gasket which prevented proper overlap of the wedges and engagement of the mechanical interlock. To overcome this problem the operator took a hammer to the interlock, bent it, and got it into position with disastrous results.

Though seemingly unrelated to autoclaves, there is a film that deals in depth with the causes of an air disaster in Paris, some years ago, involving a DC-10 aircraft. The crash was caused by a cargo door blowing open shortly after takeoff. The point in common with autoclaves is that the door closing mechanism carried a small flap which sealed a hole in the fuselage when the door was locked. The aircraft could not be pressurized unless the hole was sealed and the flap served the same function as an interlock on an autoclave. On that fateful day in Paris, the door locking mechanism did not engage properly but the operator managed to get the flap into position by applying a little extra force which bent the mechanism sufficiently and sealed the hole. Some 350 lives were lost.

A lack of adequate knowledge and understanding is dangerous, especially when working with vessels under high pressure. A very important lesson to be learned from past explosions is that providing properly constructed equipment alone will not prevent accidents. Instructions must be understood and employees must be adequately trained if potentially serious accidents are to be prevented.

Management should also establish definite operating procedures and have them prominently displayed and followed by all plant personnel. These procedures would ensure:

·         that special care be exercised in loading to prevent damage to the vessel, door gasket, and gasket bearing surfaces;

·         closing of the door must be done only by authorized personnel thoroughly acquainted with the locking mechanism and safety devices;

·         that the gasket and contact surfaces must be clean and free of any debris. Should any binding occur during closing, the trouble must be determined and corrected. Under no condition must the door or locking mechanism be forced into position;

·         that safety devices, controls, and interlocks must be checked for proper operation;

·         at the end of the opening cycle, no attempt must be made to open the door until the operator is CERTAIN that all pressure has been dissipated.

Since most of the accidents on vessels with quick-opening closures are caused by improper operation and poor maintenance, we must make certain that neither is neglected.

Safety devices are dealt within the ASME code and we do not dispute the merits of the code.

However, it should be noted that the code itself recognizes that it is impractical to write detailed requirements to cover the multiplicity of devices used for quick access, or to prevent negligent operation or the circumventing of safety devices. We, as an insurance company, have a definite preference for the inclusion among safety devices of a positive mechanical locking device that when in place ensures that the door is properly positioned and cannot be opened and when released provides visual and audible warning of any residual pressure in the vessel.

Thus far, we have dealt with the subject of ensuring safe operation of vessels with quick opening closures in terms of action required by owners and operators. What part does the B. I. & I. play in this? What do we do when we are requested to provide insurance coverage on such vessels?

Our inspection procedures call for a critical inspection of the vessel when coverage is first provided. The company sees the object through the eyes of our field inspector who must verify that the vessel is constructed to code, is free of defects, is properly installed, etc. Special attention is given to the door if it is of the quick-opening type.

The inspector describes and provides a sketch of the door and reports on the condition of wedges, wedge overlap, (if a wedge type door), condition of hinges, alignment, etc. He describes or provides a sketch showing all mechanical safety locking devices that serve to prevent accidental opening of the door under pressure and any audible or visual alarms which are fitted. He determines if the operators are competent, and if they are aware of the hazards involved. He looks at the attitude of the management and of the plant. Are they aware of the hazards? Have they issued safety instructions? Have they properly delegated responsibility for the safe operation of the vessel?

In most cases, approval of coverage hinges on the presence of a mechanical safety locking device. Our philosophy is that if an operator can see, feel or hear steam or air blowing from a vent pipe immediately after disengaging a mechanical interlock, his own common sense should tell him or her to wait until the pressure has dissipated. The device buys time. It gives them a chance to think twice.

Pressure activated devices and electrical devices do fail despite the many guarantees issued by the designers. Operators all too often regard such devices as being infallible and look to them or protection if they, the operators, make a mistake.

We feel that safety must not be compromised and cannot be overdone.

The engineering department of the B. I. & I. will not approve insurance coverage of autoclaves and similar vessels unless they are fitted with an approved mechanical locking device.

A well-known manufacturer had this to say about safety devices:

"There is a need for quick-opening doors with safety devices which remain effective even when personnel operating the door will not concern themselves about safety. We believe that, to be effective, a safety device must:

1.  be easily operated so that the operator has no reason to object to its use;

2.  be difficult to circumvent or make ineffective;

3.  be impossible to circumvent in such a manner that it can quickly and easily be returned to its original condition, say, at the end of a shift;

4.  be mounted so that supervisory personnel walking by the door can see at a glance that the device is ensuring safe operation;

5.  be simple so that the principle of operation is obvious and rugged so that confidence in its effectiveness is maintained."

The manufacturer goes on to say:

"The patented safety devices on our doors fulfill these conditions. The relative locations of the parts are fixed in our shop before shipment to ensure a safe overlap of wedges and no means of adjustment is provided. All parts are welded permanently in place. The bolt operates between lugs on the door and breech ring so that any maladjustment of the hinge would not affect the overlap ensured. The proportions of the safety device parts are adequate to resist even the full force of the hydraulic blinders tending to unlock the breech ring. As offered, the safety device should provide complete protection in that a two inch gate valve can only be closed when the door is safely locked, and the door can only be unlocked when the valve has been fully opened, and a hand wheel located in the path of the discharge from the valve has been operated.

"Various other means of providing safe operation can be envisaged but we believe that any involving electrical switches and circuits can be circumvented without difficulty and, therefore, cannot be considered prime protection. However, if required, we can supply at extra cost, in addition to our standard safety device, any equipment which may be considered to offer useful secondary protection. For example, a limit switch may be mounted on our safety device and arranged to operate only when the bolt is fully engaged; this switch could be used to control admission of steam or hot oil to the autoclave or to control the blow down valve. Also, a pressure actuated switch could be mounted on the autoclave and used to control operation of the hydraulic power unit, but the value of such an arrangement is limited by the degree of sensitivity of the pressure switches."

Finally, a few quotes from an article copied from the Autumn 1974 issue of Vigilance - The Journal of National Vulcan Engineering Insurance Group Ltd. of the United Kingdom. It is quite amazing how their experience closely parallels ours.

·         "Pressure vessels with quick-opening doors such as autoclaves, sterilizers, and vulcanizers have long been subject to the danger of explosion and from time to time doors have been blown off for various reasons such as incorrect closure, worn parts of locking devices, and improper operation."

·         "When the door is open at the end of the cycle, there may be residual pressure in the vessel which is not detectable by the pressure gauge. For example, a pressure of one pound f per in2produces a thrust of two tons on a door two meters in diameter. In these circumstances, the person opening the door may be struck and seriously injured or killed."

Let us digress for a moment and refer to the instruction manual for the Bandag type autoclaves used for curing retreaded tires. The operator is advised as follows:

Open the manually controlled chamber exhaust valve (i.e. the mechanical interlock) after the curing cycle has been completed. Never try to open the door until the chamber pressure gauge reads "0." As a double check, place your hands around the silencer of the exhaust valve to make sure all pressure had been exhausted. Even one or two psi of air over the entire surface area of the door will create enough thrust to cause damage to equipment or personnel.

·         SAFETY
The safe operation of autoclaves depends upon the following measures. Failure to implement any of them may give rise to an explosion.

1.  The provision of appropriate interlocking safety devices for the door.

2.  Periodical thorough examination of the vessel and its fittings.

3.  Arrangements for regular systematic inspection and maintenance.

4.  Conformity with proper operating procedures.

5.  Adequate training and supervision of operators.

INTERLOCKING DEVICES FOR DOORS
A test cock or other equivalent device should give an audible and visual indication of internal pressure in the vessel. This cock should also be interlocked with the door locking mechanism, in such a way that the testcock has to be completely open before the door can start to unlock.


Quick-Opening Closures

B. L. Whitley, Director
Boiler and Pressure Vessel Division
North Carolina Department of Labor

Apparently, many owners/operators and inspectors are not aware that after many months or possibly after a few years of cyclic operation, the locking mechanism on quick actuated closures and other similar devices can fail with catastrophic results without proper maintenance and care.

A close inspection of the items making up these closures include pins, bushings, bearings, bolts, nuts and wear rings. In many circumstances, these items reveal excessive wear that under certain conditions can cause the vessel to be blown open while it is under pressure. This results in injury to personnel and an untold thousands of dollars in property damage.

This problem is not unique nor is it confined to any one specific design. It is common in all types of closures having moving parts that are subject to wear and that have not been properly maintained.

Each inspector should be cautioned to examine every quick actuating device for excess wear. Surfaces of small critical parts, such as pins, bushings, bearings, etc., should be scrutinized closely for any existing abnormal conditions such as looseness, excess wear, or improper fit.

Should any of these conditions be observed, they should immediately be brought to the attention of management and the proper authorities. The inspector should also be reminded that the pressure sensing device designed to prevent the vessel from being inadvertently opened should under no circumstances be bypassed. If an inspector discovers the pressure sensing device has been bypassed, the inspector should immediately report the condition to the proper authorities.


Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.

 

 

Source: http://www.nationalboard.org/

Creep and Creep Failures

Creep and Creep Failures 

David N. French, Sc. D.
Category: Operations
 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN.  


What is creep? Creep may be defined as a time-dependent deformation at elevated temperature and constant stress. It follows, then, that a failure from such a condition is referred to as a creep failure or, occasionally, a stress rupture. The temperature at which creep begins depends on the alloy composition. For the common materials used in superheater and reheater construction, Table I (see below) gives the approximate temperatures for the onset of creep. It should be pointed out that the actual operating stress will, in part, dictate or determine the temperature at which creep begins.

The end of useful service life of the high-temperature components in a boiler (the superheater and reheater tubes and headers, for example) is usually a failure by a creep or stress-rupture mechanism. The root cause may not be elevated temperature, as fuel-ash corrosion or erosion may reduce the wall thickness so that the onset of creep and creep failures occur sooner than expected.

However, regardless of the cause, the failure will exhibit the characteristics of a creep or stress rupture. Indeed, the ASME Boiler and Pressure Vessel Code recognizes creep and creep deformation as high-temperature design limitations and provides allowable stresses for all alloys used in the creep range. One of the criteria used in the determination of these allowable stresses is 1% creep expansion, or deformation, in 100,000 hours of service. Thus, the code recognizes that over the operating life, some creep deformation is likely. And creep failures do display some deformation or tube swelling in the immediate region of the rupture.

 


Figure 1. Schematic creep curve. Courtesy Babcock & Wilcox. 

 

At elevated temperatures and stresses, much less than the high-temperature yield stress, metals undergo permanent plastic deformation called creep. Figure 1 shows a schematic creep curve for a constant load; a plot of the change in length verses time. The weight or load on the specimen is held constant for the duration of the test. There are four portions of the curve that are of interest:

  • An initial steep rate that is at least partly of elastic origin, from point "0" to point "A" in Figure 1.
     
  • This is followed by a region in which the elongation or deformation rate decreases with time, the so-called transient or primary creep, from region "A" to "B" of Figure 1. The portion from point "0" to point "B" occurs fairly quickly.
     
  • The next portion of the creep curve is the area of engineering interest, where the creep rate is almost constant. The portion from "B" to "C" is nearly linear and predictable. Depending on the load or stress, the time can be very long; two years in a test and several decades in service.
     
  • The fourth portion of the creep curve, beyond the constant-creep-rate or linear region, shows a rapidly increasing creep rate which culminates in failure. Even under constant-load test conditions, the effective stress may actually increase due to the damage that forms within the microstructure.

Without going into a detailed discussion of the atom movements involved in creep deformation, suffice it to say that creep deformation occurs by grain-boundary sliding. That is, adjacent grains or crystals move as a unit relative to each other. Thus, one of the microstructural features of a creep failure is little or no obvious deformation to individual grains along the fracture edge.

The first two stages will not leave any microstructural evidence of creep damage. Somewhere along the linear portion of Figure 1, the first microstructural evidence of damage appears as individual voids or pores. The location of these first voids or holes varies, often noted at the junction of three or more grains, occasionally at nonmetallic inclusions. These individual voids grow and link to form cracks several grains long, and finally failure occurs. The ultimate rupture is by a tensile overload when effective wall thickness is too thin to contain the steam pressure.

Since creep deformation occurs by grain-boundary sliding, the more grain boundary area, the easier creep deformation will be. Creep deformation and creep strength are a grain-size sensitive property. Thus a larger grain size improves creep strength. For austenitic stainless steels, SA213 TP321H for example, the code requires a grain size of #7 or coarser, to assure adequate creep strength. The elevated temperatures where creep occurs lead to other microstructural changes. Creep damage and microstructural degradation occur simultaneously. For carbon steels and carbon-1/2 molybdenum steels, iron carbide will decompose into graphite. For the low-alloy steels of T-11 and T-22, the carbide phase spheroidizes. Thus, creep failures will include the degraded microstructures of graphite or spheroidized carbides along with the grain-boundary voids and cracks characteristic of these high-temperature, long-time failures.

While creep failures are expected for superheaters and reheaters operating at design conditions, deviations from these parameters will promote early failures. The steam temperature always varies some from individual tube to tube, and the design allows for this variability. However, when the range of temperatures is larger than accounted for, the hottest tubes fail sooner than expected. A more likely cause of premature failure is the slow increase in tube-metal temperatures due to the formation of the steam-side scale.

Steam reacts with steel to form iron oxide along the ID surface of the tube.

The microstructures themselves will show the grain-boundary sliding and the resultant creep cracks or voids. For stainless steels, the microstructures are similar in that the failure is by grain-boundary-sliding and crack formation.

In a superheater or reheater tube, often the very first sign of creep damage is longitudinal cracks in the steam-side scale. As creep deformation expands the tube diameter, the brittle ID scale cannot follow the expansion. Cracks develop in an axial or longitudinal direction which is perpendicular to the principle hoop stress. With time, the tube continues to expand, and these cracks widen. This wide crack shortens the path from steam to steel; iron oxide forms preferentially at the tip of the crack, as there is less oxide thickness to protect the steel; and a cusp forms within the steel tube. The cusp acts as a notch or a stress raiser, reducing the local wall thickness. Creep voids form here, often before any other obvious grain-boundary damage appears elsewhere within the microstructure. With continued high-temperature operation, creep cracks grow from the cusp and ultimately weaken the cross section to the point where failure occurs.

Creep failures are characterized by:

o    bulging or blisters in the tube
 

o    thick-edged fractures often with very little obvious ductility
 

o    longitudinal "stress cracks" in either or both ID and OD oxide scales
 

o    external or internal oxide-scale thicknesses that suggest higher-than-expected temperatures
 

o    intergranular voids and cracks in the microstructure

 


Editor's note: Some ASME Boiler and Pressure Vessel Code requirements may have changed because of advances in material technology and/or actual experience. The reader is cautioned to refer to the latest edition and addenda of the ASME Boiler and Pressure Vessel Code for current requirements.

 

Table I
Initial Creep Temperature

For superheaters and reheaters, the scale that forms is essentially magnetite alloyed with chromium, molybdenum, manganese, and silicon from the alloy steels of T-11 and T-22. For waterwalls, the iron oxide may be contaminated with impurities from the boiler water and corrosion debris from the pre-boiler circuits of condenser and feedwater heaters. In any event, the thermal conductivity of the steamside scale is about 5% of the thermal conductivity of the steel tube. Thus, an effective insulating layer forms and prevents proper cooling of the tube metal by the steam. The net effect of the scale is to raise the tubemetal temperature. Depending on the scale thickness, which is dependent on the time and temperature of operation, tube-metal temperature increases of 25 - 75oF are likely. Such a large increase raises tubemetal temperatures beyond the safe design range. These elevated temperatures result in increased creep deformation rates, more rapid oxidation and corrosion (thinner walls and higher stress) and hasten the onset of creep failures. An increase of 60oF (from 1040oF to 1100oF for example) will decrease creep life by 90%. An increase of 60oF due to steam-side scale formation in a superheater or reheater is not unusual.

Carbon steel.......................

800oF

Carbon + 1/2 Molybdenum............

850oF

1-1/4 Chromium-1/2 Molybdenum......

950oF

2-1 /4 Chromium-1 Molybdenum.......

1000oF

Stainless steel....................

1050oF

 

 

 

Source: http://www.nationalboard.org/

Basic Weld Inspection

Basic Weld Inspection


John Hoh
National Board

Category: Design/Fabrication

Summary:  This article was originally published in the Winter 2010 National Board BULLETIN as the second of a two-part series. This is a continuation of the article Basic Weld Inspection - Part 1 originally published in the Fall 2009 edition of the BULLETIN, with more examples and tips the inspector can use as a guide. Some of the items in Part 2 may seem to be outside the realm of weld inspection but, when taken in context with the overall objective, they are relevant. 


 Note: The purpose of this article is to provide inspectors with a general knowledge of weld inspection. It is by no means intended to compare with the Certified Welding Inspector (CWI) requirements of the American Welding Society (AWS).

Part -1:

Weld inspection begins long before the first welding arc is struck. The inspector must review the job package to become familiar with the:

  • welding processes to be used;
  • materials and any special properties;
  • joint configurations and preparation;
  • welding procedure specifications to be used and any limitations;
  • qualifications of welders to be used and any limitations;
  • heat treatment (pre-heat or postweld), if any;
  • nondestructive examination (NDE), if any; and
  • specific ASME Code or NBIC requirements (for example, Section VIII, Div. 1, lethal service).

While not imperative, the inspector should learn to read common weld symbols such as the AWS symbols. At the very least, the inspector should always carry a reference guide to interpret weld symbols. Having reviewed all this information in advance, the inspector will be prepared to recognize any problems as they develop rather than after-the-fact.

The following examples and tips are practical applications the inspector can use as a guide.

1.  The manufacturer or repair organization (certificate holder) has indicated on the job drawing that a weld joint is to be prepared with a 60-degree bevel and root gap of 1/16 inch. Unless the bevels are milled on precision machinery, it is doubtful they will achieve an exact 60-degree bevel as indicated. The easiest solution for the certificate holder is to allow a range of plus or minus a few degrees of the target value. The same holds true for a root gap dimension with no plus or minus tolerance. Even the best welder will have difficulty maintaining an exact root gap dimension. Providing a plus or minus tolerance will make the welder’s job much easier.

2.  The inspector can use scraps of weld filler wire or rods as a gauge to quickly identify root gaps that are beyond the tolerance range. For example, if the target root gap is 3/32 inch plus or minus 1/32 inch, the inspector should be able to insert a 1/16-inch wire into the gap with little or no resistance. Likewise a 1/8-inch wire should exhibit no side-to-side movement across the gap. Real world situations are rarely this convenient, but the inspector can develop a sense of “too tight” or “too loose” with experience.

3.  The certificate holder has designed a simple nozzle to be welded to a flat head (Fig. 1). The nozzle axis is 90° to the flat head, and the attachment weld includes a 3/8-inch fillet weld. The inspector can easily measure the fillet weld to ensure compliance. Now, let’s install the same nozzle in a small diameter vessel shell (Fig. 2). The fillet weld will tend to spread or flatten on opposite sides of the nozzle due to the curvature of the shell. The inspector will need to ensure that the certificate holder has deposited enough weld to meet the design criteria. This example becomes even more critical if the nozzle is installed at an angle other than 90° (Fig. 3).

4.  Using the same nozzle attachment example as described above, let’s look at the weld joint preparation. The certificate holder has specified a 45-degree bevel around the circumference of the hole in the flat head and the vessel shell. Again, the flat head will be very easy to measure, since there is a single plane of reference (Fig. 4). The curved shell will present more of a challenge. The inspector will have to determine if the certificate holder is referencing the bevel from the vertical axis of the nozzle (Fig. 5) or from the variable reference plane of the curved shell (Fig. 6).

5.  When bevels are prepared with a cutting torch and finished with a grinder, it is very difficult to maintain an exact angle. This is why allowing a plus or minus tolerance is so important. Even obtaining a perfectly circular hole when using a torch and grinder is difficult. Fixtures are available which attach to the torch to aid in cutting circular holes and bevels, but the setup is sometimes inconvenient.

6.  A certificate holder is preparing to weld several hundred circumferential joints in power boiler tubes. ASME Section I requires these welds to be full penetration, but due to the diameter, thickness, and location in the boiler, radiography of the welds is not required (PW-41, Table PW-11). How does the inspector ensure compliance with the code? Inspectors are trained to believe only what their eyes tell them; but when the inspector cannot see the inner surface of the tube, it becomes difficult to accept that situation. This is when the inspector must take what some would call a “leap of faith.” If the tube ends are properly prepared (beveled) and a qualified welder is using a qualified welding procedure, the odds are very good that the welds will be full penetration. Does this mean the inspector should just accept all this at face value and walk away? Absolutely not! If the inspector is unfamiliar with this certificate holder’s welding procedures and welders, the inspector has the right – and duty – to witness a few of the welds being made to ensure code compliance. One “red flag” to a potential problem would be if the inspector observes that the tube ends have not been beveled. The inspector should immediately ask the certificate holder about this situation. It could be as simple as the certificate holder having just not performed that step in the process yet, or it could be as bad as his or her having tried to save time and money by not beveling the ends. From a practical standpoint, it is extremely difficult, if not impossible, to obtain a full penetration weld when the tube ends are not beveled. The welder would need to start with a large root gap and then be very careful not to “push through” excess filler metal to cause weld build-up on the inside of the tube.

 Part -2:

7.  A pressure vessel manufacturer is manufacturing a lethal service vessel. ASME Section VIII, Div. 1, paragraph UW-2 (a)(1)(d) states that all Category D joints shall be full penetration welds. That means the weld metal must extend completely from one face of the joint to the opposing face of the joint. Without watching the entire welding process, how does the inspector ensure the manufacturer has complied with Code requirements? A review of the welding procedure and any supplementary instructions combined with a verification of the joint preparation will tell the inspector much of the story. If the full penetration weld is to be accomplished by welding from both sides, the inspector should make a point of observing how the root of the first weld is prepared for incorporating the weld on the opposing face. This is usually done by mechanical means (such as grinding or chipping) or thermal gouging.

8.  When welding in areas with limited access to move, welders will sometimes shorten SMAW welding rods and GTAW filler wire. To shorten the SMAW rod, the welder will grip the rod in the electrode holder a few inches from the bare end and crumble the flux until he or she is able to grip a bare portion of the rod. When this is done, the rod identification is usually destroyed since it is normally printed on the flux close to the bare end. GTAW filler wire normally comes in 36-inch lengths with identification on one or both ends of the wire in the form of a flag-type label or embossing. A welder will seldom attempt to use a full length of wire because the free end may hit an obstruction or in some way impede the welder’s manipulation of the wire in the weld puddle. A welder may cut the length of filler wire in two or more pieces to make it easier to handle. Depending upon how the filler wire is marked, there could be one or more pieces without identification. If the certificate holder is using only one type and size of SMAW rod or GTAW wire (such as 3/32 in. E7018 or ER70S-6), the inspector may feel more comfortable if rods or wire with missing identification are found at the welder’s station. However, most certificate holders use more than one type and size of rod or wire, and the inspector must always ensure there are adequate controls in place to maintain rod or wire identification.

9.  SMAW welding rod storage seems to always stir up a lively debate. The rod manufacturer’s recommendations should always be followed or, at the very least, the rods should be stored in compliance with the information found in ASME Section II, Part C. As an example, SFA-5.1, Annex 6.11 and SFA-5.5, Annex 6.12 discuss moisture content and conditioning for carbon steel and low-alloy steel electrodes (rods). One interesting point found in these references deals with rods such as E6010 with cellulosic coverings (flux). They actually need a moisture level of approximately 3 to 7 percent to operate properly. That means if these rods are stored in a heated oven, they may be too dry to use. I have personally seen E6010 rods taken from an oven, and the flux crumbles and falls off with the slightest touch. To the other extreme, I have seen a welder quickly dip an E6010 rod in a bucket of water immediately before striking an arc. This was on plate steel in a non-pressure boundary application so there were no ASME or NBIC violation concerns, but I am sure it exceeded the rod manufacturer’s recommended moisture content. This is definitely not condoned or recommended.

10.              Holding ovens for welding rods are commercially available in many sizes. Human resourcefulness has also converted derelict refrigerators into makeshift holding ovens by installing light bulbs as the heat source. Is that permitted? As far as I know, it is not prohibited. The key, in my opinion, is the ability to achieve and maintain the recommended temperature. For example, SFA-5.1, Annex Table A3 shows a temperature range of 50°F – 250°F above ambient temperature for E7018 rods. It should not be difficult to obtain 50°F above ambient temperature during the winter in a shop where the temperature is 60°F. But, go to a shop in Louisiana or Florida in the summer, and the ambient temperature may easily be over 100°F. Can a simple light bulb in an old refrigerator achieve the necessary temperature in those conditions? There are variables such as the wattage and number of light bulbs in addition to how well the old refrigerator is insulated and sealed. As part of their normal monitoring duties, inspectors should be verifying the rod storage conditions no matter if a commercial oven is used or if a homemade alternative is in place.

11.              While we are on the subject of welding rod storage, it seems that there are always a few people who mistake holding ovens with drying or rebaking. Looking at the table below, we find E7018 should be held or stored at 50°F – 250°F above ambient temperature. If the rod flux may have absorbed excess moisture, then it may be reconditioned by drying or rebaking. That requires a temperature of 500°F – 800°F for 1-2 hours for E7018. Looking at the specifications for one manufacturer of electrode ovens, their holding ovens are capable of 550°F plus or minus 25°. That would just barely meet the minimum rebaking temperature specified in Table A3. The same manufacturer offers another purpose-built oven capable of reaching 800°F. The two big differences in their construction are the electric heating elements and the insulation thickness.

As you can see, weld inspection includes much more than just looking at the finished product. The best advice for an inspector is to stop for a moment and think about every element which goes into making a weld. That can become the inspector’s checklist for review, inspection, and verification.

 

                                                TABLE A3
Typical Storage and Drying Conditions for Covered Arc Welding Electrodes

AWS Classification

Storage Conditions(1,2)

Drying Conditions(3)

A5.1

A5.1M

Ambient Air

Holding Ovens

E6010, E6011

E4310, E4311

Ambient temperature

Not recommended

Not recommended

E6012, E6013
E6019, E6020,
E6022, E6027,
E7014, E7024,
E7027

E4312, E4313
E4319, E4320,
E4322, E4327,
E4914, E4924,
E4927

80ºF ± 20ºF
[30ºC ± 10ºC]
50% max.
relative humidity

20ºF to 40ºF
[10ºC to 20ºC]
above ambient
temperature

275ºF ± 25ºF
[135ºC ± 15ºC]
1 hr at temperature

E6018, E7015
E7016, E7018,
E7028, E7018M,
E7048

E4318, E4915
E4916, E4918,
E4928, E4918M,
E4948

Not recommended

50ºF to 250ºF
[30ºC to 140ºC]
above ambient
temperature

500ºF to 800ºF
[260ºC to 425ºC]
1-2 hr at
temperature

Notes:

(1) After removal from manufacturer's packaging.
(2) Some of these electrode classifications may be designated as meeting low moisture absorbing requirements.
(3) Because of inherent differences in covering composition, the manufacturers should be consulted for the exact drying conditions.

Table and Notes reprinted from ASME 2007 BPVC, Section II-Part C, by permission of the American Society of Mechanical Engineers. All rights reserved.

  

 

 

Source: http://www.nationalboard.org/

[MW:34820] RE: 34813] Clarification in Rate of heating and cooling.

Hello,   Please see the response below.   Regards.   P. Goswami, P. Eng, IWE.   From: materials-welding@googlegroups.com <materials-weld...