What is heat treatment?
Heat treating is the way the properties of steels and many other materials are manipulated to give the best possible performance. The results of heat treatment vary depending on the composition, and something as simple as a change in material vendor or even batch-to-batch variations can lead to ineffective heat treatment. In our heat treatment analysis practice, we have seen many cases of improper or ineffective heat treatment leading to failure. We know how critical proper heat treatment is and the right methods to assure it’s performed properly.
‘Heat treatment’ can be broken down into several commonly used practices:
Stress Relieving – The most commonly used form of heat treatment that inspectors will see, stress relieving is primarily used on welded and forged components to reduce some of the stresses imposed by fabrication. For example, when a weld cools off it constricts, putting very high tensile stresses, especially in the area at the edge of the weld known as the heat-affected zone (HAZ). Without stress relief, these areas are subject to stress-corrosion cracking and other failure mechanisms that can severely limit the usable life. Controlled cooling is equally as important in many cases to ensure the heat treatment achieves the desired properties.
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.
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.
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 post weld 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 post weld 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.
Are you looking to heat treat large components, lots of parts, or on-site heat treating?
That’s not our focus, but we are happy to recommend several good ones based your need if you contact us.
Do you need to perform heat treating analysis or perform metallurgical analysis in which heat treatment is one part of the project?
You’ve reached the right place! Our metallurgical experts can heat treat your parts to temperatures up to 2400 degrees F, and (often just as importantly) we have the expertise to help you with the analysis to make your heat treatment work well.
Do you have concerns with the quality of heat-treated parts or problems with a heat-treating vendor?
We have worked on 100’s of heat-treatment related projects, often working with vendors to help them improve their results. G2MT Labs is well-equipped to analyze the microstructure to determine the current condition of any material in the heat treatment that it experienced.