One of the most important factors determining the strength and toughness of steel forgings is grain size. Generally speaking, smaller grains resist impacts and crack propagation better and are, therefore, sought after in parts intended for demanding applications.
Toughness is also affected by grain shape and orientation. By manipulating these, a part made from steel can be given properties that vary depending on the direction of the loads applied. This deformation is achieved through forging, which creates strong parts through optimizing grain flow. However, forging alone is seldom enough to achieve the desired properties, which is why it’s used in conjunction with heat treatment.
Common Heat Treatments Used in Forging
Steel is composed primarily of iron and a small proportion of carbon. At high temperatures (1,380 to 1,800 degrees Fahrenheit), iron transitions to a face-centered cubic (FCC) structure known as austenite. This phase is capable of holding more carbon than the lower temperature, body-centered cubic (BCC) ferrite structure (the exact temperature at which the transition occurs varies based on carbon content).
Small austenite grains give the steel both ductility and resistance to crack propagation. When steel is left to cool in air, these grains grow larger and transition back to the ferrite phase. If it cools rapidly, the austenite transforms into martensite, which is both hard and brittle.
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Steel structure is mapped in a phase composition diagram, which indicates the temperatures at which these transitions occur. Heat treatment is about using this information to lock in a particular structure to deliver desired properties. There are many ways of doing this, but four of the most widely used are:
- Annealing
- Quenching & Tempering
- Normalizing
- Induction Hardening
Annealing
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The annealing process involves heating the forged part in a furnace to the temperature indicated on the phase diagram and allowing it to soak. This removes internal stresses and sees the growth of new grains. The temperature is then slowly lowered, enabling the desired phase transition to take place.
This heat treatment method is used to lower hardness, increase ductility, relieve internal stresses, and ensure a more uniform grain structure, especially because uneven cooling rates after hot forging leads to differences in grain size between thick and thin sections. And it’s often performed before machining forgings because it makes it easier to mill mounting surfaces flat, drill boltholes, and turn diameters. It also prevents the distortion that could otherwise occur as residual stresses become unbalanced.
Quenching & Tempering
Quenching refers to removing a forged part from the furnace while it’s soaking at a high temperature and cooling it rapidly by dunking it in water or oil. This is done to increase hardness by raising martensite levels.
The downside of quenching is that it creates high levels of internal stress and results in a part that’s brittle and liable to break under impacts or load. So, this stress must be relieved, which is achieved by the second step of the process: tempering.
In tempering, the forged part is heated again, but to a temperature below that used for hardening. It’s then allowed to soak before being removed and left to cool in air. This lowers the hardness from that achieved by quenching but removes internal stress and increases ductility. The overall effect of quenching and tempering is to raise the toughness of the forged part.
Normalizing
This heat treatment is similar to annealing in that it lowers internal stresses. Where it differs is that normalizing involves cooling in air rather than a controlled temperature reduction in the furnace, resulting in a higher strength and ductility than achieved by annealing.
However, the downside of air cooling is less control over grain size and phase composition since the cooling rate is determined by mass and local surface area-to-volume ratios. This means properties will vary between thinner and thicker regions of the part.
Induction Hardening
Annealing, quenching, tempering, and normalizing can be performed on batches of parts in furnaces, but induction hardening is only done one part at a time.
Induction hardening involves raising the temperature of the surface of the part, or often just selected regions of the surface, by inducing an alternating magnetic field. This is done by placing the region being hardened inside an induction coil or by scanning over the surface with a coil.
Once the magnetic field is removed, the part is either quenched or left to air cool. Both methods create a martensitic surface layer up to 0.12 inches deep over the unaltered core material, which raises surface hardness and creates desirable compressive stresses. It also increases wear resistance while retaining the tensile properties of the substrate.
When Heat Treatments Are Recommended
Forged parts are often very hard, thanks to the grain flow the process causes, but may not have the combination of mechanical properties sought in the finished component. They also tend to be challenging to machine.
The most important effects of heat treatments in forging are to improve machinability and optimize toughness, strength, and hardness. Many forged parts undergo a series of heat treatments, such as annealing, followed by quenching and tempering after milling and turning. Others may be normalized, which is less expensive than annealing, to remove residual stress and perhaps have selected regions induction hardened to improve durability.
Learn More From Trenton Forging
Trenton Forging is an industry-leading, U.S.-based forging company that specializes in impression die forging. We’re capable of producing symmetrical and asymmetrical forgings from stainless steel, microalloyed steel, alloy steel, and carbon steel. In addition to forging, our in-house capabilities include machining, heat treatments, quality testing, reverse engineering, and tooling design.
Please contact us today to learn more about our capabilities or visit our blog for more forging resources.
