Heat Treatment in Metal Fabrication: Gases and More

Heat treatment involves subjecting a metallic material or component in the solid state to a controlled thermal cycle. This controlled thermal treatment involves heating, holding at an elevated temperature (soaking), and then cooling. Heat treatment may be carried out in one or more heating and cooling cycles.

These treatments are performed to effect certain microstructural changes in a material. These changes, in turn, result in desirable mechanical and physical properties. In short, heat treatment is used to modify, restore or enhance the properties of metals and alloys.

Industrial heat treatment can entail simple, relatively low-temperature treatments to help heat treatment operators achieve high-quality welds in components and to relieve the stresses locked into the structure from previous forming or welding operations. However, heat treatment can also involve complicated higher-temperature processes to give specific levels of mechanical properties after fabrication of the components is complete. Certain post-treatment steps may also be necessary to remove heat treatment defects.

There are many different heat treatment processes. Some of these are carried out in different heat treatment furnace types, such as heat treatment nitriding furnaces or carburizing furnaces.

• Preheating is applied to parent metal components before welding begins to prevent the material welded from cooling too quickly post-welding.
• In the case of multi-pass welding procedures, interpass temperature control may be applied.
• Post-weld heat treatment (PWHT) may be used to extend preheat and interpass temperatures to the completed weld, to relieve residual stresses, or restore/enhance mechanical or corrosion properties.
• Tempering is applied to a hardened or mechanically worked material to modify and soften any hard, brittle phases.
• Annealing is carried out to recrystallize and soften materials. It improves ductility but reduces tensile strength. Normalizing is a form of annealing but is only used to treat steel.
• Solution heat treatment is used to fully soften a solid solution alloy system.
• Also known as precipitation hardening, age hardening heat treatment is used to strengthen solid solution alloys.
• Quenching involves rapidly cooling the material from an elevated temperature - often by using cryogenic gases.
• Commonly applied to carbon and alloy steels, stress relief entails heating and soaking the whole component to reduce residual stresses induced by welding or cold work. 

Solution treatment involves the heating of a component to a high temperature in order to dissolve second-phase particles and compounds present in the microstructure into solid solution. The alloy is heated to a sufficiently high temperature so that elements or compounds present in the microstructure are taken into 'solid solution'. Quenching, in other words, rapid cooling from high temperatures, is used to retain these elements or compounds in solution. Frequently, an age hardening treatment is used after solution treatment to re-harden the alloy microstructure in a controlled manner.

Powder processes commonly used in heat treatment include atomization, powder metallurgy sintering, and hot isostatic pressing (HIP). Like most heat treatment processes, industrial gases are essential for these powder metallurgy treatments.

The parent material close to a weld often undergoes some changes during welding. To control these effects and improve the properties of the welded joint and the surrounding material after welding, heat treatment may be applied at three distinct stages, namely preheating, interpass temperature control and post-weld heat treatment (PWHT). The final PWHT step may simply extend the preheat and interpass temperature to the completed weld, or it may be an elevated temperature treatment designed to relieve residual stresses or to bring about certain metallurgical changes across the whole weldment.