STORY OF STEEL PART 7 – HEAT TREATMENT

Earlier in our “Story of Steel” we began to look at some metallurgical terms that covered constituents that form during the welding cycle. For example, we looked at ferrite, austenite and pearlite and we saw how these microstructures form and change, from the liquid pool beneath the heat of the arc and through the cooling cycle during welding.

As practitioners and supervisors, it is possible that during the time you are involved in the art and science of joining, that you will be involved in producing welds that will eventually need to be heat treated on completion of welding. One of the many great things about steel is the ability to move through various temperatures and improve or change properties during so called “heat treatment” operations. 

There are several types of heat treatment that can be applied to steel and these can be listed as:

1. Post Weld Stress Relief

2. Normalizing 

3. Annealing

4. Quench and Tempering

The most likely form of heat treatment you will experience after you complete welding of the job is the first item listed, the one we define as Post Weld Stress Relief. 

It is a known fact that stress builds up during the thermal expansion and contraction of a component during welding. These are otherwise known as residual stresses. Post Weld Stress Relief is a method whose objective is to reduce and redistribute these residual stresses that have been introduced by welding.

This kind of post weld stress relief treatment is often specified for welded fabrications for several reasons, these include:

• gaining dimensional stability for post weld machining, 

• softening of any hardened (martensitic) areas, 

• possible improvement of crack resistance in the weldment. 

A stress relief treatment is designed so that it does not cause any changes in the microstructure of the steel. We know, from our previous parts of this Steel story, that metallurgical changes will not occur until we exceed the magic temperature of 723 Deg C where changes will begin. This heat treatment is therefore carried out below this temperature, as shown in the shaded area in Figure 1 below. 

Figure 1. Temperature range for stress relief of steel components.

The extent of relief of the residual stresses depends on the steel type and composition, the temperature and the holding time at that temperature. A commonly used guideline for Post Weld Stress Relief is that the joint should be soaked at peak temperature for one hour for each 25mm (1 inch) of thickness. Typically, a temperature of about 625°C is used and components are held at this temperature for a period of time dependent on thickness. To avoid distortion and reintroducing additional stresses, the heating and cooling rates are very slow.

Requirements for stress relief are called for in various standards such as ASME, for pressure containing equipment, and in Section 5.0 of CSA W 59 for the fabrication of steel structural components. The ASME code normally requires stress relief for weldments in the 19-32 mm thickness range and above, depending on the code section being used for the design and build. 

The CSA W 59 structural fabrication standard specifically states that Post Weld Stress Relief may be required where:

• welded assemblies must retain dimensional stability during machining 

• where corrosion (namely stress corrosion) may be involved during service. 

These requirements will be reflected in the Welding Procedure Specification (WPS) or the Welding Procedure Data Sheet (WPDS) that would define the total fabrication sequence.

Another heat treatment that you may come in contact with is the second one on the list, “Normalizing” after completion of welding. This heat treatment is usually driven by design where a particular toughness may be required and a fine grain size is required in the finished weldment in order to achieve this. It entails heating the steel into the “fully austenitic” region of the phase diagram to allow recrystallization as depicted in the shaded area in Figure 2 below. The temperature is controlled at just above the austenizing temperature for a soaking period and then cooled in air. This relatively rapid cooling produces a fine ferrite/pearlite grain size which will exhibit improved toughness. 

Figure 2 Temperature Range for Normalizing followed by an Air Cool

This is again a fantastic property of the wonder material we call steel; we can effectively renew the microstructure completely by reheating from room temperature into the shaded area in Figure 2 and then controlling the cooling back to room temperature.

The other two heat treatments mentioned above, annealing and quenching and tempering, are considered to be rarer requirements for weldments and, as such, will not be covered in this article on our “Story of Steel”.