In the field of mechanical manufacturing, forgings play a pivotal role. Whether they are key components of automotive engines or precision parts used in the aerospace industry, their performance and quality depend directly on the quality of the forgings themselves. Heat treatment, as a crucial stage in the production of forgings, has a decisive influence on their final performance. An appropriate or inappropriate heat treatment process can make a significant difference. This article takes an in-depth look at common problems encountered during the heat treatment of forgings and the corresponding countermeasures, helping the forging industry improve product quality and ensure the stable operation of mechanical equipment.
When steel is heated, a loose and brittle layer of iron oxide scale forms on its surface; this phenomenon is known as oxidation. At the same time, the reduction of carbon content at the surface is referred to as decarburization. Although these two changes may seem minor, their impact on forgings is profound. Oxidation and decarburization reduce surface hardness and fatigue strength, much like coating the surface of originally solid armor with a fragile layer of paint, making it more susceptible to damage under external impact and long-term service. Moreover, they directly affect dimensional accuracy, which can be disastrous for precision forgings with extremely strict dimensional requirements.
To effectively prevent oxidation and decarburization, the forging industry has adopted various measures. Salt bath furnace heating is a common option, as the special environment inside the salt bath can isolate air to a certain extent, thereby reducing oxidation and decarburization. When higher surface quality is required, protective coatings can be applied to the forging surface, essentially dressing the forging in a “protective suit,” or heating can be carried out in a protective atmosphere or under vacuum. Although these methods increase production costs, they significantly improve surface quality and dimensional accuracy. From a long-term perspective, they are well worth the investment.
During quenching heating, the phenomenon in which austenite grains become significantly coarsened is known as overheating. If the heating temperature is excessively high, resulting in grain boundary oxidation and the onset of partial melting, the phenomenon is called overburning. Overheated forgings suffer a marked decline in performance, especially in toughness. This is akin to planting a “time bomb” within the internal structure of the steel, making it prone to quenching deformation and cracking during subsequent use due to insufficient toughness.
While an overheated structure can be corrected through normalizing, overburned forgings must be scrapped, representing a significant waste of resources. Therefore, strict control of heating temperature and holding time is essential to prevent overheating and overburning. Forging operators must precisely set heating parameters according to the type of steel and the shape of the forging, and closely monitor temperature changes during heating to ensure the entire process remains within a controllable range.
During quenching and cooling, temperature differences between various parts of the forging and differences in phase transformation generate stresses known as quenching internal stresses. When quenching stress exceeds the yield strength of the steel, deformation occurs; when it exceeds the tensile strength, cracks form and the forging becomes scrap. This is like triggering an “earthquake” inside the forging, causing uncontrollable structural changes.
To prevent deformation and cracking, different quenching methods and process design measures can be adopted. Techniques such as step quenching or isothermal quenching can effectively control temperature changes during quenching and reduce internal stress. From a design perspective, structures should be as symmetrical as possible, with uniform tapers and no sharp corners, as such optimizations can help reduce quenching stress. In addition, timely tempering after quenching is indispensable, as tempering relieves part of the internal stress and stabilizes the internal structure of the forging.
Insufficient hardness in forgings may result from factors such as low heating temperature, inadequate holding time, insufficient cooling rate, or surface decarburization. This is comparable to an “incomplete refinement” of the steel, preventing it from reaching the required strength and hardness standards. Fortunately, this issue is not irreparable. Re-quenching can eliminate insufficient hardness, but prior to re-quenching, annealing or normalizing is required to restore the internal structure of the steel and create favorable conditions for re-quenching.
When addressing insufficient hardness, it is important to distinguish between hardenability and the actual effective hardened depth of a forging. Hardenability is an inherent property of steel that determines the extent to which it can be hardened during quenching. The actual effective hardened depth of a forging, however, is influenced by multiple factors, including shape, size, and cooling medium. This means that even the same steel can exhibit different effective hardened depths under different forging shapes and cooling conditions. Therefore, in practice, quenching effectiveness should not be judged solely on steel hardenability; other factors must also be considered.
At the same time, hardenability should not be confused with hardness potential (quench hardness). Hardenability refers to the depth of hardening, while hardness potential is the maximum hardness that steel can achieve after quenching, which mainly depends on the carbon content of martensite. Steel with good hardenability does not necessarily have high hardness potential. For example, low-carbon alloy steels often have good hardenability but low hardness potential, whereas high-carbon steels have high hardness potential but poor hardenability. This is similar to two people with different strengths: one adapts well to various environments (good hardenability) but lacks explosive power (low hardness potential), while the other has great explosive power (high hardness potential) but limited adaptability (poor hardenability). Understanding these differences helps in making more rational decisions when selecting materials and designing heat treatment processes.
Quenching distortion is a troublesome issue in heat treatment and can be divided into volume distortion and shape distortion. Differences in specific volume among various microstructures before and after quenching are the main causes of volume changes. For example, when a workpiece with an initial pearlitic structure transforms into martensite during quenching, its volume expands. Conversely, a large amount of retained austenite may cause volume shrinkage. For high-precision components, uniform volume expansion leading to dimensional changes requires special attention.
Shape distortion is more complex and includes bending of plates or shafts, expansion or contraction of internal holes, and changes in hole spacing. Distortion arises from many factors, such as uneven heating temperature, improper placement in the furnace, release of residual internal stress at high temperature, and asynchronous cooling during quenching. Components with complex shapes are particularly prone to deformation during quenching due to their structural characteristics.
To reduce distortion, various rational heat treatment processes can be applied. These include lowering quenching heating temperature, slow heating or preheating, using static heating methods, proper bundling and suspension of workpieces, selecting appropriate quenching immersion methods based on component shape, and adopting step quenching or isothermal quenching. In addition, intentional pre-deformation in the opposite direction prior to quenching can be used to offset post-quenching distortion. This method requires precise design and operation based on the component;s shape characteristics and deformation law.
In component design, symmetry should be emphasized and large differences in cross-section should be avoided to reduce distortion caused by uneven cooling. For groove-shaped or open components prone to distortion, they can be made into closed structures before quenching, for example, by adding ribs at the groove opening, and then cut open after quenching. Measures such as adding process holes, adopting composite structures, and selecting appropriate steels can also effectively reduce distortion. For instance, micro-distortion steels can be selected for dies and molds with high precision and strict distortion limits, while pre-hardened steels are suitable for high-precision plastic molds.
For distortion that occurs after heat treatment, various straightening methods can be used. Cold press straightening is suitable for shaft components with hardness below 35 HRC. Hot spot straightening is applicable to components with hardness above 35–40 HRC. Hot straightening utilizes the good plasticity of austenite and transformation superplasticity. Tempering straightening combines external force with tempering treatment. Impact straightening induces plastic deformation by striking concave areas. Hole shrinking treatment uses thermal stress to contract expanded holes. Each method has its own applicable range and operational considerations and must be selected according to specific conditions.
Quenching cracking is one of the most severe defects in heat treatment. It occurs when heat treatment stress exceeds the fracture strength of the material. Cracks often appear in discontinuous chains, with traces of quenching oil or brine on the fracture surface, no oxidation color, and no decarburization on either side of the crack. Quenching cracks can result from a wide range of causes, including poor material management, improper cooling, insufficient hardness at the core of incompletely quenched components, components with the most dangerous quenching crack dimensions, severe surface decarburization, excessively high quenching temperature, lack of intermediate annealing before repeated quenching, lack of preheating or excessively rapid heating of large-section high-alloy steel components, poor original microstructure, raw material defects, expansion of forging cracks during quenching, overburning cracks, fixture quenching of low-hardenability steels, premature cleaning before step-quenched components cool to room temperature, cryogenic treatment, and failure to temper promptly after quenching.
Although these causes seem complex, they ultimately stem from unreasonable heat treatment processes or inherent material issues. To prevent quenching cracking, strict control is required in material management, process design, heating control, and cooling methods. Ensuring proper material selection, uniform heating, appropriate cooling rates, and timely tempering are all effective preventive measures. At the same time, rigorous inspection of raw material quality is essential to prevent defective materials from entering production.
Heat treatment of forgings is a complex and delicate process involving numerous process parameters and operational details. Issues such as oxidation, decarburization, overheating, overburning, quenching internal stress, insufficient hardness, quenching distortion, and quenching cracking all require focused attention and effective solutions. By continuously optimizing heat treatment processes and strictly controlling every stage of production, the quality of forgings can be significantly improved, enabling them to deliver stable and reliable performance in various mechanical applications. Looking ahead, with ongoing technological advancement and accumulated experience, the forging industry will be better equipped to address these challenges, advance to higher levels, and provide stronger support for the development of the mechanical manufacturing industry.
