Common defects in heat treatment and their prevention

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The hardness is not up to standard. There is an empirical relationship between the hardness of metal materials and their static tensile strength as well as fatigue strength; moreover, it is also related to certain processing properties of metals such as their cold formability, machinability, and weldability； Since hardness testing does not damage the workpiece, is simple to perform, and provides straightforward data, it is widely used as the most important quality inspection criterion for heat-treated workpieces; in many cases, it is even the only technical requirement. Unsatisfactory hardness is one of the most common defects in heat treatment. It is mainly caused by factors such as insufficient hardness, inadequate quenching cooling rate, surface decarburization, insufficient hardenability of the steel, excessive residual austenite after quenching, and insufficient tempering. The phenomenon of lower hardness in local areas of a quenched workpiece is called soft spots. The surrounding tissue in the soft zone is mostly a mixture of martensite and troostite distributed along the original austenite grain boundaries. Soft spots or uneven hardness are usually caused by uneven quenching heating or uneven quenching cooling. Uneven furnace temperature during heating, as well as insufficient heating temperature or holding time, are the main reasons for uneven heating. Uneven cooling is mainly caused by bubbles of the quenching medium adhering to the workpiece surface during cooling, contamination of the quenching medium (such as oil droplets in water), or insufficient agitation of the quenching medium. Furthermore, the steel structure is too coarse, with severe segregation; large carbides or large amounts of free ferrite can also lead to uneven quenching and the formation of soft spots. Soft spot: The purpose of quenching heating is to enable the workpiece to undergo structural transformation during the quenching process. To do this, it must be heated to an appropriate temperature and held at that temperature for a sufficient time. The low heating temperature and insufficient holding time prevented the original pearlitic structure from being completely transformed into austenite, and the austenite formed as a result was uneven in composition; consequently, a fully martensitic structure could not be obtained after quenching, leading to the formation of soft spots in the workpiece after quenching. 1- Fine-grained martensite 2- Quenched troostite 3- Pearlite. When the quenching medium is not stirred adequately, or when the workpiece moves insufficiently within the medium or enters it at an incorrect angle, it often delays the breakdown of the vapor film in certain areas on the surface of the workpiece. This results in a reduced cooling rate in those areas, leading to the formation of high-temperature decomposition products and the creation of soft spots or localized reductions in hardness. The water vapor film is more stable than saltwater, so soft spots form more easily on water-quenched workpieces. The higher the temperature of water and aqueous solutions, the easier it is for soft spots to form. Carbon steels with poor hardenability are prone to soft spots when the workpiece cross-section is large. If the surface of the workpiece is not clean, with rust, carbon black, etc., it can also lead to lower hardness after quenching. Insufficient hardness; inadequate heating often leads to insufficient hardness in quenched parts. However, improper cooling is a common cause of insufficient hardness in the workpiece. An excessively long pre-cooling period between taking the workpiece out of the furnace and starting quenching, an inappropriate choice of cooling medium, or excessive control of the cooling medium’s temperature can result in insufficient cooling capacity. Oxide scales or salt deposits on the workpiece surface, as well as a high temperature of the workpiece when it is removed from the quenching medium after quenching, can all cause the supercooled austenite to decompose in the pearlite transformation region of the C curve, leading to the formation of non-martensitic structures such as sorbite and troostite, which results in insufficient hardness of the workpiece. The presence of a large amount of residual austenite in the quenched microstructure is a major reason for the insufficient hardness of quenched workpieces. The amount of residual austenite is related to the chemical composition of austenite; when the carbon content is greater than 0.5%–0.6%, residual austenite can be clearly observed in the quenched structure. As the carbon content increases further, the amount of residual austenite rises sharply, and at a carbon mass fraction of 1.4%, the volume fraction of residual austenite reaches 30%. Any alloying element that is solidly dissolved in austenite by substitution causes an increase in the amount of residual austenite. When the amount of residual austenite is low, there is no significant effect on hardness; however, when the amount of residual austenite is high, it leads to a decrease in hardness. A residual austenite content of 20% by volume results in a reduction in quenched hardness of about 6.5 HRC. The soft spots and insufficient hardness in workpieces subjected to high-frequency quenching and carburizing include two types: residual soft spots where the surface layer has not been fully hardened, and depth-related soft spots resulting from uneven depths of the hardened layer. These hardness defects are caused by factors such as improper material selection, poor original microstructure, and inappropriate electrical parameters for high-frequency quenching heating, as well as issues with the inductor and cooling devices. High-frequency quenching is commonly used for medium-carbon structural steels and low-carbon medium-alloy structural steels. Since heating in high-frequency quenching occurs rapidly, the carbon in austenite does not have enough time to become fully homogenized through diffusion. Therefore, in steels containing carbide-forming elements such as Cr, Mo, W, and V, the phase transition temperature is relatively high; as a result, soft spots and uneven hardness can occur during high-frequency induction heating quenching. When selecting steel for high-frequency quenching, it is necessary to ensure that the content of these elements does not exceed certain limits. The type, morphology, size, and distribution of carbides in steel have a significant impact on the quality of parts quenched at high frequencies. When there are network-like carbides in the steel, or when the carbide sizes are too large and unevenly distributed, defects such as uneven hardness and insufficient hardness can occur. Therefore, high-frequency quenching is greatly influenced by the pre-treatment, and the optimal initial microstructure for high-frequency quenching is tempered sorbite resulting from quenching and tempering. When the high-frequency induction coil is uneven, it can also result in insufficient hardening. An improper spraying angle, as well as unreasonable sizes, numbers, and positions of the spraying holes, or clogged spraying holes, often lead to insufficient hardness in the parts subjected to high-frequency quenching or the formation of soft spots. Insufficient hardness and numerous soft spots in carburized workpieces are mainly caused by inadequate carburizing, decarburization during quenching, too low quenching temperature, insufficient quenching cooling rate, excessive residual austenite on the surface, over-tempering, unclean workpiece surfaces, uneven carburizing, or uneven cooling. The mechanical properties of non-ferrous metal alloys are not up to standard. The most widely used non-ferrous metals in industry are aluminum, copper, magnesium, titanium, and their alloys. Non-ferrous metals share the same heat treatment principles as steel, but they have their own characteristics. For example, the eutectoid transformation plays an important role in the heat treatment of steel, but it is rarely encountered in non-ferrous metals ； Martensitic transformation is the primary method used to strengthen steel materials, but with a few copper and titanium alloys aside, other non-ferrous metals generally cannot be strengthened through martensitic transformation. The common heat treatment processes for non-ferrous metals are homogenization annealing, recrystallization annealing, stress relief annealing, solution treatment, and aging treatment. Solution aging is the most common and important heat treatment strengthening process for non-ferrous metals. Special attention should be paid to the following issues in the heat treatment of non-ferrous metals: Non-ferrous metals are reactive and have strict requirements regarding the heating environment. For example, the heating environment for titanium alloys should generally be a vacuum or a slightly oxidizing atmosphere ； To avoid oxidation, magnesium alloys are often heated in a protective atmosphere of sulfur dioxide or carbon dioxide ； To avoid hydrogen embrittlement, red copper needs to be heat-treated in a neutral or weakly oxidizing atmosphere. To achieve the maximum solubilization effect, the solubilization temperature of many non-ferrous metal alloys is close to the solidus temperature; to prevent overheating and over-sintering, it is necessary to strictly control the furnace temperature and the heating and holding time. Overheating and overburning: In the heat treatment process, when metals or alloys are heated to excessively high temperatures, their grain sizes grow very large, resulting in a significant decline in their properties; this phenomenon is known as overheating ； When the heating temperature approaches its solidus line, the phenomena of grain boundary oxidation and partial melting occur, which is known as overburning. Overheating: Overheated tissues include coarse grains in structural steel, large martensite particles, excessive residual austenite, Widmanstätten structure; networked carbides in high-speed steel, eutectic structures (ledeburite structure), naphthalene-shaped fracture surfaces; excessive ferrite in martensitic stainless steel; whitish-gray surfaces on brass alloys due to zinc loss, and a pitted surface after pickling. The typical superheated structure is shown in Figure 2. Based on the ease with which it can be eliminated through normal heat treatment processes, overheated tissues can be divided into two categories: stable overheating and unstable overheating. Generally, overheated tissues that can be removed through normal heat treatment are referred to as unstable overheated tissues. Stable overheated structure refers to the overheated structure that cannot be completely eliminated by ordinary normalizing, annealing, and quenching. A key characteristic of overheating is the coarsening of grains, which reduces the yield strength, plasticity, impact toughness, and fatigue strength of steel, while increasing its brittle transition temperature. Another important characteristic of overheating is the coarsening of the quenched martensite, which reduces impact toughness and wear resistance, as well as increasing the tendency for quenching deformation and quenching cracks. Other defects associated with overheating in steel include Widmanstätten structure, naphthalene-shaped fracture patterns, and stone-like fracture patterns; these not only diminish the mechanical properties of the steel but also increase the likelihood of quenching cracks. To prevent overheating, a proper heat treatment process must be designed and implemented carefully, with strict control over furnace temperature and holding time. Overheated microstructures can generally be eliminated through multiple annealing or normalizing processes. For more severe cases of overheating, such as those resulting in a stone-like fracture pattern, heat treatment alone is not sufficient; high-temperature deformation combined with annealing is required to eliminate such conditions. Overheating: Overheated tissue is characterized by local melting at the grain boundaries, fibrous voids, blackening and bubbling on the surface of aluminum alloys, as well as a gray and dull fracture surface; in magnesium alloys, it manifests as oxidative tumors on the surface. Overheated tissue severely deteriorates properties and makes heat treatment cracks highly likely to occur; therefore, overheating is a heat treatment defect that must not be allowed. Once overheating takes place, valuable parts can only be discarded, so it is essential to strictly prevent overheating in heat treatment processes. The spheroidization level is not up to standard. Cars, tractors, and various other machines make extensive use of standard components and fasteners. Standard components such as shafts, pins, and rods are mostly manufactured by using automatic lathes, while fasteners such as bolts, nuts, and rivets are primarily produced through cold heading. To improve productivity and to accommodate automatic cutting and cold heading processes, the pre-treatment of the steel material involves annealing or spheroidizing annealing; it is therefore necessary to control the degree of spheroidization. Automatic turning processes require steel to have good turning properties; its plasticity should not be too high, as this can lead to the material sticking to the cutting tool and continuous generation of swarf. It is desirable that the steel structure consist of flake-shaped pearlite ； Cold heading processing requires that the steel possess good cold heading properties and high plasticity, in order to prevent cracking during the process; it is also desirable that the metallographic structure of the steel be spherical pearlite. To this end, the industry standard JB/T5074-91 \"Grades of Spheroidization Rate for Low and Medium Carbon Steels\" is used to evaluate the spheroidization grade. Rated by the degree of carbide spheroidization, grade 1 corresponds to a spheroidization rate of 0, meaning the pearlite is entirely flaky, while grade 6 corresponds to a spheroidization rate of 100%, meaning the carbides are fully spheroidal. Medium-carbon steel for cold heading generally requires grades 4 to 6, while low- and medium-carbon steel for machining on automatic machines generally requires grades 1 to 3. Poor spheroidization during the pre-heat treatment of low and medium carbon steels has a significant negative impact on their cold heading and automatic cutting properties. The effect of the spheroidization level on cold heading performance is shown in Table 7. It can be seen that when the spheroidization level is 1–3, cracking occurs during cold heading, whereas no cracking takes place at levels 4–6. Therefore, a spheroidization level of 4–6 is considered acceptable for steels used in cold heading. Extensive production experience shows that when the spheroidization degree of steel balls used in automated turning processes is controlled at level 1 to 3, the surface roughness of the parts is appropriate, the wear of band saws is normal, and productivity is high ； If the spheroidization rate exceeds grade 3, it is difficult to carry out machining using automated machine tools.