Quenching cracks

Quenching is mainly used for steel parts. The process involves heating the steel to a temperature above the critical temperature Ac3 (for hypoeutectoid steel) or Ac1 (for hypereutectoid steel). The steel is held at this temperature for a period to achieve complete or partial austenitization. It is then rapidly cooled at a rate greater than the critical cooling rate to below Ms (martensitic transformation initiation temperature), or isothermally near Ms, to undergo martensitic or bainitic transformation.

Quenching cracks occur during quenching or during subsequent room-temperature storage after quenching; the latter is also called aging cracks. The distribution of cracks is not fixed, but they generally tend to form at sharp corners and abrupt changes in the cross-section of the workpiece. The fundamental cause of quenching cracking is that tensile stress exceeds the material’s fracture strength; even if it does not, cracking can still occur due to internal defects. There are many specific causes of quenching cracking, and analysis should differentiate them based on crack characteristics.

 

The inherent brittleness of martensite is the primary cause of quenching cracks, while its crystal structure, chemical composition, and metallurgical defects are influencing factors. The magnitude, direction, and distribution of macroscopic internal stresses induced by various process conditions and part dimensions and shapes are external factors in quenching cracks. The following analysis will examine quenching cracks in steel parts from microscopic to macroscopic perspectives, from the inside out.

It is well known that medium- and high-carbon steels, after quenching, exhibit low toughness and high brittleness, making them prone to microcracks and macroscopic cracking. This is mainly determined by the inherent brittleness of martensite. The inherent brittleness of martensite, in turn, is determined by the metallurgical quality of the material, carbon content, and alloying elements, the original microstructure, the martensitic microstructure, microstress, and microcracks.

 

 

 

 

Figure 1. Macroscopic morphology of quenching cracks

 

Shrinkage cavities and severe rolling defects cause significant material inhomogeneity. This makes the material unsuitable for heat treatment. Many metallurgical defects can interact with macroscopic or microscopic stresses and trigger quenching cracks. Such metallurgical problems include segregation, dissolved hydrogen, forging or rolling defects, inclusions, and ferrite-pearlite or carbide-banded structures.

 

 

 

Figure 2. Quenching cracks propagating along inclusions

 

Increasing carbon content reduces the fracture strength of martensite. According to the theory of brittle solids, fracture strength:

E and d values are linked to carbon content. Increased carbon content lowers the interatomic bonding force in martensite, decreases the elastic modulus, and reduces steel fracture strength. It also raises the d value, further reducing fracture strength.

The effects of alloying elements on quenching cracks vary. For example, elements such as Mn, Cr, V, and Mo, like C, exhibit increased crack susceptibility with increasing content. Boron (B) is distinctive because it enhances hardenability. Research on rare-earth elements and quenching cracks is limited and yields differing conclusions. Suitable concentrations of rare-earth elements reduce the frictional force on dislocation motion, thereby decreasing the tendency toward brittle fracture. Rare-earth elements, when they accumulate at grain boundaries, purify and strengthen these areas, hindering impurity accumulation, such as phosphorus (P), and potentially mitigating intergranular fracture.

In addition to the steel’s chemical composition, the original microstructure before quenching also has a significant impact. For example, lamellar pearlite, non-equilibrium structures such as martensite and bainite, inhomogeneous, network carbides, non-metallic inclusions, forging overheating structures, and streamlines can all lead to or promote quenching cracks.

 

 

Figure 3. Influence of different pearlite morphologies on quenching cracks

1-Fine lamellar pearlite; 2-Point-like pearlite; 3-Fine-grained pearlite; 4-Coarse-grained pearlite

 

Microcracks readily form during the martensitic transformation, particularly in medium- and high-carbon steels, whereas they rarely form in the martensitic structure of low-carbon steels. This is attributable to the parallel lath morphology in low-carbon martensite, which limits intersection and, together with high plasticity, facilitates stress relaxation through deformation, reducing the probability of microcrack development. In contrast, high-carbon martensite is characterized by frequent intersections of martensite plates and limited deformation capacity in its lamellar structure, leading to localized stress fields at contact points. When the local stress exceeds a critical value, microcracks form. These microcracks are inherent defects that exacerbate the brittleness of high-carbon martensite, and under additional applied stress, may propagate into macrocracks.

If the chemical composition, metallurgical quality, and original microstructure of the materials are the same, but the size and shape of the parts differ, then under the same heat-treatment conditions, they will still exhibit different quenching-crack tendencies. For ordinary steel, excessively thin or coarse workpieces generally will not crack during quenching. Hisashige Owa believes that during water quenching, the critical diameter is precisely the size at which quenching cracking occurs. The critical diameter is the diameter at which the core of the workpiece, when cooled in a given quenching medium, reaches 50% martensite.

 

Figure 4 Relationship between Critical Diameter DⅠ, Carbon Content, and Quenching Cracks

 

Quenching cracking is closely tied to the workpiece’s shape. The steel part’s shape affects quenching stress magnitude and distribution. Notches, sharp corners, grooves, cavities, or sudden cross-section changes concentrate internal stress. These areas are prone to fracture.

Heating temperature, holding time, and heating equipment all affect the occurrence of quenching cracks.

A higher quenching temperature increases the risk of cracks. Higher temperatures and longer holding times cause austenite grain growth. This leads to coarser, more brittle martensite and lowers fracture strength. That is the main reason for increased cracks. Vacuum furnaces are least likely to cause cracks. Next are electric furnaces, salt-bath furnaces, and, finally, flame furnaces. Flame furnaces, like heavy-oil- or coal-fired units, pose the highest risk.

 

 

Figure 5: Relationship between the number of samples with quenching cracks and quenching temperature

(a) T10 steel quenched in water; (b) 9CrSi steel quenched in water and oil

 

During quenching cooling, the cooling rate must be carefully controlled within two temperature ranges. One region is the critical zone, requiring rapid cooling for complete hardening. To harden the part, rapid cooling is necessary in this critical zone. The other region is the low-temperature zone, prone to quenching cracks, below the MS point. In this temperature range, austenite transforms into martensite, causing volume expansion and generating type II distortion, type II stress, and macroscopic heat treatment stress, which can lead to quenching cracks; therefore, this is called the danger zone. In the danger zone, cooling should be as slow as possible to mitigate quenching internal stresses.

 

 

Figure 6: Schematic diagram of the quenching critical zone and danger zone

 

After quenching, parts are often machined. According to the nature of the machining, it can be divided into three categories: hot working, mechanical processing, and chemical processing, as well as their combined applications. The formation of cracks due to post-quenching machining results from the interaction among macroscopic and microscopic internal stresses, microcracks induced by quenching, and the loads or internal stresses that develop during post-quenching machining.

 

Figure 7. Schematic Diagram of Processing After Quenching of Parts

 

Quenching cracking of steel parts is influenced by factors such as the martensitic microstructure, quenching-induced internal stress, workpiece size and shape, and production conditions. Identifying these influencing factors enables taking corresponding measures to prevent quenching cracking. This section mainly discusses methods to prevent quenching cracking of steel parts, which significantly affect production practice, from the perspectives of steel selection, quenching component design, reasonable formulation of heat-treatment technical conditions, selection of quenching media, and selection of quenching cooling methods.

Different steels have different probabilities of quenching cracking. Generally speaking, the higher the carbon, Cr, and Mo content of the steel, the more likely it is to crack during quenching. The following figure shows the relationship between the tendency to crack during water quenching and the steel’s chemical composition. The more negative the index shown in the figure, the greater the tendency to crack during quenching. Since various steels exhibit different quenching-cracking tendencies, when designing parts, steel should be comprehensively analyzed and selected based on performance requirements, hardenability, and brittleness/hardness, with process and economic factors taken into account.

 

 

 

 

Figure 8. Relationship between chemical composition and quenching cracks (water quenching)

 

The design of mechanical parts often focuses primarily on the material’s mechanical properties, neglecting the performance of heat-treatment processes. Some parts may seem reasonable in terms of material strength, but from a heat-treatment perspective, their shape and size may be inappropriate. To prevent cracking during quenching and rapid cooling, efforts should be made to ensure uniform heating and cooling, as well as uniform contraction and expansion. Therefore, two points should be considered in part design: (1) the cross-section should be uniform; (2) there should be no notch effect. Good design requires uniform cross-sectional thickness, symmetrical shape, smooth transition, and the addition of process holes. For large concave dies with complex shapes and large dimensions (greater than 400mm), and for thin, long convex dies, a separate inlay structure should be adopted to simplify the complex shape, reduce the large size, and convert the inner surface of the die to the outer surface. This facilitates both hot and cold processing, effectively reduces the tendency to quench cracks, and improves the product qualification rate.

 

 

 

 

Figure 9. Improvements in the design of workpieces with non-uniform cross-sections

(a) Combined design and machining holes; (b) Design of through holes; (c) Design of uniform cross-sections

 

Designers should rationally formulate the technical conditions for heat treatment based on the parts’ working conditions and performance requirements. As long as the working requirements are met, the degree and location of quenching hardening should be minimized. High hardness and overall quenching should not be pursued blindly; instead, local and surface hardening should be used to replace overall hardening, thereby reducing the risk of quenching cracks.

Quenching media consist of various substances in three states: solid, liquid, and gas. The following factors should be considered when selecting a quenching medium: (1) Cooling capacity of the quenching medium; (2) Influence on distortion cracking; (3) Economy and durability; (4) Safety and reliability, etc.

The cooling curve of an ideal quenching medium is shown in the figure below. This medium has the highest cooling capacity at the temperature at which supercooled austenite decomposes most rapidly, while its cooling capacity becomes more moderate near the martensite start (Ms). This ensures the hardening requirements are met, reduces quenching stress, and prevents quenching distortion cracking. The stability of supercooled austenite varies among different steels, and the actual workpiece dimensions also differ, necessitating the selection of different quenching media. Although many types of quenching media are available, no quencher can simultaneously suit all steels and workpiece sizes. It is essential to select the most suitable medium based on specific circumstances and in conjunction with various quenching cooling methods.

 

 

Figure 10: C-curve and ideal cooling intensity

 

Generally speaking, quenching cracks occur in the hardened portion. To achieve hardening, rapid cooling from the austenitizing temperature at a rate greater than the critical cooling rate is necessary. Whether the sum of thermal stress and phase transformation stress is tensile (positive) or compressive (negative) determines whether quenching cracks occur. A positive value increases the likelihood of cracking, while a negative value decreases it. To prevent quenching cracks, thermal stress should be fully and effectively utilized while reducing phase transformation stress.

 

 

Figure 11: Relationship between cooling rate and quenching cracks

 

(1) Pre-cooling quenching: The workpiece is removed from its austenitizing temperature and pre-cooled in air for a period of time to reduce the temperature difference between different parts. Alternatively, if technical conditions permit, some non-martensitic structures can form at the thinnest cross-section or at corners before full quenching.

(2) Two-liquid quenching: From the perspective of simply preventing quenching cracks, the key to two-liquid quenching is the slow cooling effect of the second-stage quenching medium. Strong cooling is applied first, followed by weak cooling, such as water-oil, water-air, or oil-air.

(3) Stage quenching: Stage quenching involves rapidly cooling the workpiece directly from the quenching temperature to a temperature above Ms, holding it at that temperature for an appropriate time, and then air-cooling it. For high-carbon steel with large cross-sections that are prone to deformation and cracking, two to three stages of stage quenching should be used. (4) Isothermal quenching: The workpiece is cooled from the quenching temperature to a temperature slightly above the Ms point at a cooling rate greater than the critical quenching rate, and held at that temperature for a relatively long time to allow the supercooled austenite to transform into bainite. Oil quenching is generally used.

In addition, there are thin-shell quenching, intermittent quenching, local quenching, and water temperature adjustment methods.

Furthermore, the rationality of each process before quenching, the determination of heating parameters, and tempering are also effective methods to prevent quenching cracks in steel parts.

 

There are many reasons for quenching cracks and distortions in parts. Once the above defects occur, the following aspects should be analyzed:

(1) Whether the material selection and structural design of the parts are reasonable.

(2) Whether there are defects in the raw materials or blanks.

(3) Inspecting the heat treatment process.