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Acoustic Emission Examination of Metal Pressure Vessels
Anatomy of a Catastrophic Boiler Accident
Austenitic Stainless Steel
Auto-Refrigeration
Black Liquor Recovery Boilers - An Introduction
Boiler Efficiency and Steam Quality: The Challenge of Creating Quality Steam Using Existing Boiler Efficiencies
Boiler Logs Can Reduce Accidents
Boiler/Burner Combustion Air Supply Requirements and Maintenance
Carbon Monoxide Poisoning Preventable With Complete Inspection
Combustion Air Requirements:The Forgotten Element In Boiler Rooms
Creep and Creep Failures
Description of Construction and Inspection Procedure for Steam Locomotive and Fire Tube Boilers
Ensuring Safe Operation Of Vessels With Quick-Opening Closures
Environmental Heat Exchangers
Factors Affecting Inservice Cracking of Weld Zone in Corrosive Service
Failure Avoidance in Welded Fabrication
Finite Element Analysis of Pressure Vessels
Fuel Ash Corrosion
Fuel Firing Apparatus - Natural Gas
Grain Boundaries
Heat Treatment - What Is It?
How to Destroy a Boiler -- Part 1
How to Destroy a Boiler -- Part 2
How to Destroy a Boiler -- Part 3
Identifying Pressure Vessel Nozzle Problems
Inspection, Repair, and Alteration of Yankee Dryers
Inspection, What Better Place to Begin
Laminations Led to Incident
Lay-up of Heating Boilers
Liquid Penetrant Examination
Low Voltage Short Circuiting-GMAW
Low Water Cut-Off Technology
Low-Water Cutoff: A Maintenance Must
Magnetic Particle Examination
Maintaining Proper Boiler Inspections Through Proper Relationships
Microstructural Degradation
Miracle Fluid?
Organizing A Vessel, Tank, and Piping Inspection Program
Paper Machine Failure Investigation: Inspection Requirements Should Be Changed For Dryer Can
Pipe Support Performance as It Applies to Power Plant Safety and Reliability
Polymer Use for Boilers and Pressure Vessels
Pressure Vessels: Analyzing Change
Preventing Corrosion Under Insulation
Preventing Steam/Condensate System Accidents
Proper Boiler Care Makes Good Business Sense:Safety Precautions for Drycleaning Businesses
Putting a Stop to Steam Kettle Failure
Quick Actuating Closures
Quick-Actuating Door Failures
Real-Time Radioscopic Examination
Recommendations For A Safe Boiler Room
Recovering Boiler Systems After A Flood
Rendering Plants Require Safety
Residential Water Heater Safety
School Boiler Maintenance Programs: How Safe Are The Children?
Secondary Low-Water Fuel Cutoff Probe: Is It as Safe as You Think?
Short-Term High Temperature Failures
Specification of Rupture Disk Burst Pressure
Steam Traps Affect Boiler Plant Efficiency
Stress Corrosion Cracking of Steel in Liquefied Ammonia Service - A Recapitulation
Suggested Daily Boiler Log Program
Suggested Maintenance Log Program
System Design, Specifications, Operation, and Inspection of Deaerators
Tack Welding
Temperature And Pressure Relief Valves Often Overlooked
Temperature Considerations for Pressure Relief Valve Application
The Authorized Inspector's Responsibility for Dimensional Inspection
The Effects of Erosion-Corrosion on Power Plant Piping
The Forgotten Boiler That Suddenly Isn't
The Trend of Boiler/Pressure Vessel Incidents: On the Decline?
The Use of Pressure Vessels for Human Occupancy in Clinical Hyberbaric Medicine
Thermally Induced Stress Cycling (Thermal Shock) in Firetube Boilers
Typical Improper Repairs of Safety Valves
Wasted Superheat Converted to Hot, Sanitary Water
Water Maintenance Essential to Prevent Boiler Scaling
Water Still Flashes to Steam at 212
Welding Consideration for Pressure Relief Valves
Welding Symbols: A Useful System or Undecipherable Hieroglyphics?
What Should You Do Before Starting Boilers After Summer Lay-Up?
Why? A Question for All Inspectors


Heat Treatment - What Is It?


J.G. Gillissie

October 1981   

Category: Design/Fabrication 

Summary: The following article is a part of National Board Classic Series and it was published in the National Board BULLETIN. (4 printed pages)


A short time ago during a joint review of an ASME Certificate Holder, I found myself asking the question, "Do you use heat treatment?"

The immediate answer was, "Oh yes."

I have asked the same question many hundreds of times in a like number of fabricators' shops, knowing full well that my question was all-inclusive and covered a number of processes. Ninety-five times out of any hundred the answer I got was a straight "yes" or "no." Once in a blue moon the company representative would explain that he uses only stress relieving of weldments for those material P-numbers and material thicknesses as required by the Code sections to which he is fabricating.

When I get an answer like that I think to myself, "This gent knows what he's talking about." At the same time, I have a deep suspicion that HE is thinking to HIMSELF, "This clown probably doesn't know the difference between stress relieving and stealing third base."

Generalized questions usually get generalized answers. As an example, if somebody asks me the question, "Do you travel?", my answer would probably be, "Yes." If I were asked, "How do you travel?", my answer would possibly be, "By airplane, train and automobile but not by bicycle, pogostick or horseback."

Get the point? There is a difference. There is nothing wrong with the term, "heat treatment," but it is a generalized term covering various processes. Heat treatment in any of its forms is used to achieve a desirable improvement in the characteristics of material or to regain those characteristics which may have been adversely affected by production processes such as welding/bending/forming etc.

Let's take a short look at some of the most frequently used processes of heat treatment, those which the Authorized Inspector may encounter in boiler and pressure vessel fabrication shops.

STRESS RELIEVING (postweld heat treatment)

This is by far the most frequently used form of heat treatment which will confront the authorized inspector. As a result of welding processes used to join metals together, the base materials near the weldment, the deposited weld metal and, in particular, the heat affected zones transform through various metallurgical phases. Depending upon the chemistry of the metals in these areas, hardening occurs in various degrees, dependent mainly upon carbon content. Again, this is particularly true in the heat affected zone (HAZ) adjacent to the weld metal deposit where the highest stresses due to melting and solidification result. Stress relieving, as the name implies, is designed to relieve a proportion of these imposed stresses by reducing the hardness and increasing ductility, thus reducing danger of cracking in the vessel weldments.

The Code sections contain requirements for stress relieving, specifying rate of heating and cooling above 800oF and requiring a holding temperature, usually one hour per inch of thickness of the material. The holding temperatures vary with the P-numbers of the material which in turn are based on alloy content. As an example, P-1 through P-4 require 1100-F holding temperature, P-1 being carbon steels, P-3 being carbon steels alloyed in relatively small percent with molybdemum, manganese and vanadium. P-4 steels are the nickel steels, chrome-molys and nickel- chrome-molys. P-5, P-6 and P-7 high alloy steels generally require a higher holding temperature ranging up to 1350oF. Some of the special steels now listed in the Code sections call for even higher temperatures.

Following the holding (soaking) time, controlled cooling down to 800oF or lower is vitally important. Many high carbon steels are subject to surface cracking if cooled too rapidly.

QUENCHING AND TEMPERING

Oriented toward carbide steels such as carbon-moly, this process is designed to enhance toughness as well as controlling yield strength and ultimate tensile strength of steel. The steel is heated to above its upper critical temperature and quickly immersed in fresh water or brine to achieve rapid setting of the desired metallurgical structure. Oil quenching is sometimes used. The usual practice is to quench until cooling reaches around 800oF, quickly followed by a tempering period in a fired furnace in order to soften the martensitic structure and achieve the desired mechanical properties in the material including a desired measure of ductility. The tempering process is, in effort, a stress relieving process.

NORMALIZING AND TEMPERING

This process is used for virtually the same purposes as quenching and tempering. It differs in that normalizing is accomplished by cooling in air in place of fast quenching in a liquid. Air normalizing, much slower than liquid quenching, may be used by itself or the material may be subjected to a controlled furnace tempering process in order to better control desired mechanical properties.

Steel manufacturers will furnish material in either of the above conditions when so specified on the purchase order or as required by the material specification.

As a cautionary note; alloyed steel mechanical properties are ultimately determined by the tempering process and if the materials are subsequently welded during fabrication, subsequent stress relieving temperature, if used, should not exceed that of the tempering process, otherwise mechanical properties of the material may be adversely affected.

SOLUTION HEAT TREATMENT (solution annealing)

While the Code sections state that heat treatment of austenitic stainless steel (P-8) is neither required nor prohibited, this refers to postweld stress relieving. There are certain processes to which this material may be subjected. These are performed almost exclusively by the material manufacturers due to the fact that temperature ranges and holding time are critical and require careful controls, otherwise damage to the material can result from either too high or too low a furnace temperature. Material manufacturers have the metallurgical staffs to determine requirements.

In solution heat treatment the material is subjected to a high heat, around 2000oF, and rapidly cooled in liquid in order to achieve an evenly distributed solution of carbon and austenite in the metallurgical structure of the material.

STABILIZING HEAT TREATMENT

Everything said in the first paragraph under solution heat treatment also applies to stabilizing heat treatment. In the latter process the material is cooled slowly in order to bring as much carbon as possible out of solution and into evenly distributed concentrations apart from the austenite.

Both solution heat treatment and stabilizing heat treatment are used to reduce susceptibility to intergranular stress corrosion and embrittlement also to increase high temperature creep strength.

PREHEATING

While most of us do not look upon preheating as a form of heat treatment, its use can contribute substantially in reducing hardness in all three constituents of a weldment; the parent metal, the weld metal deposit and the heat affected zone. As a weldment cools, it goes through various transformations in which molecules rearrange themselves. If cooling is rapid, this rearrangement is arrested resulting in entrapment of stresses and hardening of the material with coincident loss of ductility which is the highly desirable ability of the material to bend elastically, under stress.

Preheating of the weldment area achieves better weld penetration and slows the cooling process, thus allowing added relief of stresses and reduced hardening of the materials.

The ASME Code sections take cognizance of the foregoing, in some cases allowing exemption from postweld stress relieving PROVIDED preheating of a specified temperature is used.

Here again, a word of caution is in order. Preheat, like any other heat treatment, must be carefully planned and used. Specific written procedures should be provided for each individual use. Misuse, such as light surface heating, can do more harm than good. A soaking heat and maintenance of interpass temperature throughout the weldment - and beyond, are recommended.

In all cases, high chrome-moly steels should be preheated prior to welding and postweld stress relieved at around 1400oF.

In summary, the authorized inspector (or ANI) is not assigned the duty of being an authority on metallurgy of all the various ferrous and nonferrous materials used in boiler, pressure vessel or piping system fabrication. The various Code sections do, however, require that results of heat treatment be made available to him for his review in order that he may assure himself that temperature readings and holding (soaking) time conform with Code requirements. Only a diligent study of Code requirements will enable him so make this decision.

As previously mentioned, heat treatments which will confront the AI-ANI are for the most part preheating and postweld heat treatment, that is, stress relieving.

Some points to remember:

Post weld heat treatment is designed to return a metal as near as possible to its prefabrication state of yield, ultimate tensile and ductility.

The rate of temperature rise, holding time at temperature and rate of cooling are vitally important. For this reason, furnace thermocouples must measure metal temperature, not furnace atmospheric temperature.

Heat treatment of any type must be a planned, systematic action. Poorly performed heat treatment can result in far more harm to material than any good which may result.

Test coupons must be subjected to the identical conditions as the vessel or part in order to obtain meaningful tensile and toughness (Charpy) test results.

The foregoing is a short generalization. Specific requirements are found in ASME Section II "Material Specifications" and in the "Material Tables", of the various Code sections.


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.







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