Classification of Heat Treatment Cracks

Quenching cracks—longitudinal cracks (microstructural stress type), arc cracks (local tensile stress type), quenching cracks in large workpieces (longitudinal and transverse fractures), surface cracks along edges and contours (local tensile stress type), decarburization cracks, and Type II stress cracks.

Quenching cracks —— Longitudinal cracks (tissue stress type), arc cracks (local tensile stress type), quenching cracks in large workpieces (longitudinal and transverse fractures), surface cracks along edges and contours (local tensile stress type), decarburization cracks, and Type II stress cracks.

Longitudinal fissure

⑴ Macroscopic morphology of longitudinal fissures

Cracking initiates along the surface of slender components and, while propagating longitudinally, also extends inward into the cross-section perpendicularly to the surface, forming a wedge-shaped crack that is wide on the outside and pointed on the inside. The propagation of longitudinal cracks invariably terminates near the center of the cross-section. From an external perspective, the longitudinal single crack and the wedge-shaped crack on the cross-section represent the basic macroscopic morphology of longitudinal cracking.

⑵ Conditions for the formation of longitudinal fissures

Quenching through is a necessary condition for the formation of longitudinal cracks. After quenching through, the stress state in small workpieces is characterized by residual stresses of the microstructural type; generally, the tangential stress associated with microstructural stresses is significantly greater than the axial stress. Therefore, the development of microstructural residual stresses constitutes the stress condition conducive to the formation of longitudinal cracks.

⑶ Longitudinal crack prevention measures

① Use a slower cooling medium, such as oil. It is also possible to use dual-quenching media—water and oil—but for some small parts, dual-quenching with water and oil has no practical value.

② Avoid overheating the workpiece during heating; after removal from the furnace, allow it to cool slightly beforehand. After quenching, perform tempering promptly.

③ Strengthen technical management and technical training, and effectively provide quenching crack theory education to personnel involved in process operations.

Arc crack

⑴ Conditions for arc cracking

It should simultaneously feature a structural design that allows for rapid overall cooling, prevents through-hardening, and accommodates geometrically sensitive areas prone to arc cracking.

⑵ Structural forms of geometrically sensitive areas

There are holes, concave surfaces and bowl-shaped surfaces, abrupt changes in cross-sectional dimensions, and shaft shoulders.

⑶ The slow-cooling effect on geometrically sensitive areas

The primary function of the above-mentioned structural form during the quenching and cooling process is to significantly reduce the actual cooling rate in that region, thereby producing a slow-cooling effect.

⑷ Tissue at geometrically sensitive sites

The slow-cooling effect on geometrically sensitive areas either results in the formation of tempered martensite in locally unhardened regions, surrounded by martensite (which can be observed macroscopically or microscopically in metallographic examination), or leads to a noticeable thinning of the hardened layer in those regions. In Heat treatment Among the arc cracks generated during production, the former type accounts for the vast majority.

⑸ Formation and propagation mechanisms of arc cracks, as well as their typical macroscopic morphologies.

Arc cracks first form on the surfaces of geometrically sensitive areas and then propagate inward along curved (arc-shaped) surfaces, initially in a direction toward the interior of the cross-section. In severe cases, these cracks can penetrate through the rest of the part’s cross-section and extend outward to its surface, eventually emerging there in an arc-like pattern. In extreme cases, the affected areas may even detach along the arc cracks (or can be easily detached by tapping). The fracture surfaces typically exhibit various curved (arc-shaped) configurations; most notably, when viewed from several different directions, they all appear arc-shaped—a key indicator for identifying arc cracks. Factors that exist in geometrically sensitive areas and can induce stress concentration effects—such as sharp corners—do not trigger or promote the formation of arc cracks.

⑹ Preventive measures for arc cracks

① Implement localized strong cooling:

For parts that may cause arc cracking, consider the possibility and implementation methods for locally intensively cooling (in the high-temperature range) geometrically sensitive areas.

② Implement localized weak cooling:

For parts that may be prone to arc cracking, it is necessary to consider the possibility and implementation methods of locally weakening cooling (in the high-temperature range) at geometrically sensitive areas. A prime example is hole-plugging quenching, which slows down the cooling rate within the holes in the high-temperature zone, ensuring that the entire microstructure transforms into troostite. ③ Implement a quenching method with slow cooling in the low-temperature zone.

Quenching cracks in large parts

⑴ The residual stress in the quenching of large parts is of thermal origin.

The stronger the cooling capacity of the quenching medium, the larger the cross-sectional dimensions, and the higher the heating temperature. , The greater the residual stress from quenching.

⑵ Stress Application Mode and Crack Formation Causes

In the late stage of cooling, the outer layer of metal has already cooled to a low temperature, while the temperature of the inner metal is necessarily higher than that of the outer layer. As the inner metal continues to cool, it experiences strong constraints from the outer layer due to accompanying volume shrinkage, thereby generating three-dimensional tensile stresses in the central region. The maximum tensile stress occurs precisely at the center of the cross-section. According to the theory of metallic mechanical properties, under the action of three-dimensional tensile stresses, the metal's ability to undergo plastic deformation is severely restricted, causing it to transition into a brittle state and making it highly susceptible to low-stress brittle fracture. This is precisely the fundamental reason why the core regions of large components with pearlitic microstructure develop cracks under thermally induced stress.

⑶ Fracture characteristics

① Short cylindrical:

It often exhibits longitudinal cracking, and when the height is about twice the diameter, transverse fractures may occur. This phenomenon is commonly observed in carbon tool steels; these parts often have a network of cementite at their centers, which reduces the steel's strength and causes cracks to propagate along its path.

② Shaft-type:

When the maximum tensile stresses in the axial and tangential directions exceed the material’s strength at the center of the part, cracking first initiates at that location. Subsequently, under the influence of quenching stresses, the cracks propagate outward from the inside, both longitudinally and transversely, until they finally become visible on the outer surface. However, cracks may also terminate internally at some point, resulting in internal cracks. If the residual stresses are sufficiently high, the part may completely fracture on its own toward the end of the quenching process. More commonly, though, such cracks become apparent only after they have already appeared on the part’s surface, typically through machining or other post-processing methods. When the length of the part significantly exceeds its diameter, transverse cracks are more prevalent than longitudinal cracks; moreover, multiple transverse or longitudinal cracks may develop on the same part. The crack initiation site is usually located at the center of the cross-section. Only when metallurgical defects are present in the region near the center of the cross-section might the crack initiation site deviate from the center.

③ Gear ring type:

Typically made from medium-carbon cast steel, this material can only develop radial cracks. The crack initiation point is located at the geometric center of the cross-section or at the hot spot formed during casting. From there, the cracks propagate radially outward through the central plane of the gear ring, eventually leading to complete fracture.

④ Exploding internal cracks:

A blast is a type of cracking that poses a risk of injury and should be carefully guarded against. Blasting occurs after the cooling phase has nearly ended.

⑤ Fracture characteristics:

The fracture surface is smooth and flat, with no obvious plastic deformation occurring; it exhibits a typical brittle fracture appearance.

⑷ The role of internal metallurgical defects

In the center and vicinity of large-section components, the maximum tensile stress due to thermal stresses is concentrated and exerted. This area also serves as the site where numerous metallurgical defects originate or persist. These defects are significant factors that promote and induce cracking, and they represent both the natural crack sources and direct causes of quenching cracks in large components. Due to various constraints and influencing factors, the overall metallurgical quality of large castings and forgings in China remains far from ideal, making it one of the most important practical factors contributing to quenching cracks in such components.      It should be noted that any factor present on the surface of large parts that could induce stress concentration effects will never trigger or promote crack formation during the quenching process. Therefore, it is unnecessary to remove surface defects from large castings and forgings prior to heat treatment.

⑸ Preventive Measures for Cracking in Large-Size Quenching

① Leveraging the interaction and dual-action characteristics of basic stresses induced by heat treatment, design or improve quenching processes for large components. ② Using the method of pre-cooling and temperature reduction; ③ Quenching cooling is not carried out to low temperatures; ④ Pay attention to the tempering cooling method when performing timely tempering.

Edge crack

⑴ Formation conditions of edge cracks

① It can only occur near sharp edges or external contours;

② Under rapid quenching and cooling conditions; The two factors mentioned above result in extremely high residual stresses at the crack initiation site (rapid microstructural transformation and small temperature differences across the cross-section). Moreover, cracks typically form during the early stages of quenching, and thereafter, as cooling time extends, these cracks rapidly propagate. When developing heat treatment processes, it is essential to take this characteristic of edge-corner cracks into account.

⑵ Macroscopic characteristics of edge cracks

A single or multiple capillary cracks located near the contour or edge, running roughly parallel to it; these cracks are wider on the outside and narrower at the tip, with their orientation essentially perpendicular to the part’s outer surface and relatively shallow in depth.

⑶ The Influence of Heating Temperature and Stress Concentration Factors

① Edge cracks can form even at lower quenching temperatures; at normal quenching temperatures, they have already begun to develop and grow, and under overheating conditions, they will severely expand.

② Generally, stress concentration factors do not exert any influence; however, surface machining tool marks are an exception. On quenched parts with circular contours, edge-corner cracks that develop near the edges almost always originate and propagate along the circular machining tool marks. This is because the machining tool marks near the edges happen to fall within the region of the local combined tensile stress field that is critical for the formation of such cracks.

⑷ Preventive measures for edge cracks

① Use a milder quenching cooling medium;

② The temperature of the quenching cooling medium must not be lower than... 15℃ , when below 5℃ The crack is inevitable;

③ Strengthen personnel training and enhance the technical management of heat treatment production.

  Detachment

During the quenching of certain rotating parts (such as wheels and gears) and cylindrical parts (such as shafts and pins), sometimes a phenomenon occurs in which cracks—known as "decracking"—appear at locations such as wheel flanges, gear rims, and shaft shoulders, or even lead to the complete spalling or chipping of these areas.

⑴ The formation pattern of delamination

① Conditions for the occurrence of cleavage:     

Heat treatment conditions: Surface heating quenching of rotating parts and cylindrical parts also occurs on similar parts subjected to conventional quenching. Most cracking occurs under water-quenching conditions; oil quenching is relatively rare.

Metallographic conditions: Extensive metallographic analysis has determined that the coexistence of martensite and bainite phases in the vicinity of the region where cracking occurs is both a necessary and sufficient condition for crack formation.     

During surface hardening, the distribution of pearlitic structures in the martensitic microstructure and the original microstructure typically depends on the corresponding process conditions. By adopting process measures—such as leaving a soft zone at the ends—to eliminate differences in cross-sectional microstructures, we can remove the microstructural conditions that give rise to cracking.     

During conventional quenching, the cooling rate is typically determined by the local geometric structure's slow-cooling effect. For example, the transition area at the shoulder of a shaft often exhibits such a geometric configuration that promotes slow cooling. When the cooling capacity of the quenching medium declines due to various reasons—such as excessively high water temperature—the end faces of shaft-like components, the end faces of stepped sections, and other areas with localized geometric structures that exhibit a slow-cooling effect may develop differences in microstructure across their cross-sections, thereby leading to cracking.

The formation and propagation of decohesion both occur within the fully martensitic microstructure. In production, most instances of decohesion arise under conditions where the martensite exhibits no signs of overheating; in isolated cases, however, the martensite can also appear remarkably coarse.

⑵ The formation, propagation mechanisms, and macroscopic morphology of cracks.  

① Shoulder Separation: In the shoulder separation process of shaft and roller-type parts, cracking typically initiates near the shoulder (or edge) on the end face—often at multiple locations simultaneously (on the same circumference or on concentric circles situated relatively close to each other). The cracks then propagate inward into the cross-section in a direction perpendicular to the surface, while simultaneously expanding circumferentially to form circumferential cracks. Subsequently, the cracks change direction and continue propagating inward along the curved surface, toward the outer cylindrical surface near the shaft shoulder. Eventually, the cracks emerge onto the outer cylindrical surface, forming arc-shaped cracks.

② Gear Rim Separation: Gear rim separation initially begins with cracking at the corner where the gear rim intersects with the spoke section, then propagates inward along the cross-section (along the curved surface) and simultaneously expands circumferentially along the corner (often a sharp corner), ultimately leading to the detachment of the gear rim.

⑶ Influencing factors

① Cooling Rate: First, if the local cooling rate is excessively rapid, quenching cracks will form in the areas that cool fastest. Second, the occurrence of cracking is also linked to a significant slowdown in cooling within the high-temperature transformation zone of localized regions of the quenched part, leading to differences in microstructure across the cross-section and consequently inducing localized tensile stresses. In heat treatment practice, the reasons for the localized reduction in cooling rate are twofold: one is the inherent slow-cooling effect of the local geometric structure itself, and the other is the combined effect of operational errors during heat treatment or the interaction between operational mistakes and the local geometry. Under conditions where cross-sectional microstructural differences have already emerged, increasing the cooling capacity of the quenching medium in the low-temperature region or intensifying the cooling rate at the relevant parts of the quenched workpiece will significantly heighten the risk of crack formation.

② Chemical composition: A significant increase in carbon content or the addition of certain alloying elements to steel markedly enhances its hardenability, thereby greatly increasing its susceptibility to cracking.

Stress concentration: It is insensitive to typical stress concentrations, but excessively deep machining tool marks can significantly induce cracking.

⑷ Preventive measures for delamination  

① Leave a Soft Zone at the End: Reserving an appropriately wide, unhardened soft zone at the ends of roller and shaft components that undergo surface hardening can effectively prevent cracking from occurring.

② Select the quenching medium correctly: The difference in microstructure across the cross-section of surface-hardened parts is unavoidable. On the premise of ensuring sufficient hardening, choose a cooling medium with a milder cooling rate whenever possible.

③ Locally strong cold, but proceed with caution.

④ Eliminate surface stress concentrations: Avoid sharp corners in cross-section transitions and ensure that surface roughness is machined to a level higher than... Ra12.5。

⑤ Ensure the chemical composition and prevent excessive carbon content or residual alloy element levels.