Summary: The following article is a part of National Board
Classic Series and it was published in the National Board BULLETIN. (4
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
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
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
Steel manufacturers will furnish material in either of the above conditions
when so specified on the purchase order or as required by the material
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
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.
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
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.