Artem Komarov noted that welding can seriously affect hardened or hardened metals, depending on the hardening technology used. Work-hardened or deformation-hardened metals subjected to intense local welding pressure tend to recrystallize and soften in the heat-affected zone (HAZ). Provided the correct filler metal is used, the only affected area is the HAZ. The additive and filler metal do not undergo recrystallization and remain as strong as the base metal. This explains why, when you’re dealing with work-hardened or deformation-hardened steel, damage usually occurs in the high-risk zone next to the weld, and not directly at the joint.
This is especially true for cold rolled steel, wrought iron and drawn or rolled aluminium. When working with these materials, joint design is critical, and you must consider the amount of stress the finished product will experience during service.
Precipitation-hardened metals undergo more complex changes than factory-hardened ones, but the result is the same: the HAZ goes through an annealing cycle and becomes softer. This is since the precipitate, which gives the metal strength, increases and agglomerates when heated — it ages over time. This reduces the effect of dispersion strengthening. The higher the heat, the faster the metal reaches an aged or weakened state. Post-weld heat treatment can remedy this if you carefully select the filler metal to match the aging characteristics of the base metal.
Metals that have been solution quenched have the smallest amount of weld change. There is some grain growth along the melt line, but this is usually not enough to have any effect on the properties of the metal.
Benefits of heat treatment
Because of all this, post-weld heat treatment is often very useful in maintaining the strength of the weld, as it softens or hardens any martensite or bainite that has formed in the HAZ. It also relieves stresses that can lead to cracking.
In fact, proper heat treatment can change grain size; change ductility, hardness, toughness or tensile strength; improve magnetic or electrical properties and machinability; relieve stress; recrystallize cold-worked metals; and even change the chemical composition and surface properties of the metal (case hardening).
The main thing is to do the heat treatment correctly: it is more than just bringing the torch to the steel and then letting it cool down a bit. The critical factors in heat treatment are what you might expect: temperature, time, and cooling rate. Of course, the chemical composition of the surrounding materials also affects the efficiency.
Methods and tips for heat treatment
When it comes to controlled heating of metal, there are several ways to do this, including oxy-fuel or air-fuel torches and colored pencils with a temperature gauge, furnace heating, induction heating, electric contact heating, natural gas, or an electrically heated bath of salt or molten metal.
There are also several methods of controlled cooling, including gradual furnace cooling, still air cooling, mixed air cooling, fan cooling, water cooling, and sand cooling.
But in terms of heating and cooling, control is critical. That is, the ability to control how slowly (or quickly) the part heats up, as well as the temperature to which it is heated, how long it is held at that temperature, and how long it takes to cool to room temperature. And the specifications for all these variables depend not only on what kind of metal it is, but also on what you want to achieve with the heat treatment.
Cooling metal to room temperature.
Heat treatment equalizes the temperature throughout the metal and makes it fully austenitic. With very slow cooling, austenite transforms into ferrite and pearlite, and the metal reaches its softest state with fine grain size, good ductility, and excellent machinability.
Normalizing is another heat treatment method often used to prepare the metal for future heat treatment. Normalization can increase the uniformity of the metal’s internal structure, improve ductility, and reduce internal stresses. And although it does make the metal softer, it does not make it as soft as when fully annealed. Normalizing involves heating the metal to a temperature just above 3, holding it there until austenite forms, and then slowly cooling it in still air.
Thermal stress relief is exactly what it says: heat treatment to relieve internal stress. It involves heating the metal to a temperature below the lower transformation temperature (A1), holding it there long enough to release trapped stresses, and then slowly cooling it down. This is sometimes referred to as process annealing.
For stress reducing steel, the most common temperature range is 1100 to 1150 degrees Fahrenheit. This is enough to reduce residual yield stresses by 80 percent, yet low enough to prevent any metallurgical changes in most steels. It is possible to achieve 90% stress relief by heating the metal just below the critical temperature, but some steels can become brittle after thermal stress relief at these temperatures.
They cover the basics of how welding affects heat treated metals and some of the ways we can counter this impact with heat treatment methods, concluded Artem Komarov.
