Spring Materials and Heat Treatment

A spring is a mechanical component that leverages the elastic properties of its material. Its performance is closely tied to the material used, and there are two key requirements for spring materials: one relates to the fundamental material properties essential for the spring's intended application, while the other concerns the material's processability, which is critical during manufacturing. Ultimately, our spring production aims to fully harness both the inherent qualities of the material and the unique characteristics of the spring itself.

1. Basic requirements for spring materials:

Based on the working performance of springs, the material must exhibit sufficient elasticity, with an elastic limit high enough to meet operational demands. Moreover, since many springs endure cyclic loads during operation—where their deformation varies in response to changing forces—it is essential not only that the material possesses high elasticity but also that it boasts excellent fatigue strength, enabling it to withstand prolonged exposure to alternating loads over time. Springs are used in a remarkably wide range of applications; therefore, depending on the specific service conditions and operating environments, diverse and specialized requirements are often imposed on spring materials. For instance, spring materials may need to offer corrosion resistance, heat resistance, cold-temperature resilience, stable elasticity, non-magnetic properties, and even superior electrical conductivity. In addition to these functional demands, the material must also demonstrate outstanding machinability. In summary, ideal spring materials should be easy to process, simple to manufacture, consistently reliable in performance, and cost-effective—all while maintaining affordability.

Requirements for the service performance of spring materials;

The service performance of a material refers to the properties required to ensure the proper functioning of a spring—specifically, the characteristics it exhibits during use. High-quality mechanical products invariably demand durability, safety, lightweight design, and low energy consumption, all of which contribute to their market competitiveness. Similarly, springs—as fundamental components—face even higher expectations. To minimize machine weight as much as possible, spring materials must offer superior strength and stiffness. Meanwhile, to maximize service life, these materials need exceptional impact resistance, fatigue performance, anti-relaxation capabilities, and excellent corrosion resistance.

Mechanical Properties:

According to the primary failure modes of springs—primarily fracture (most of which are fatigue fractures) and stress relaxation (deformation)—the mechanical performance requirements for spring materials mainly include the following aspects:

Strength Performance:

The spring material is required to have a high elastic limit, proportional limit, strength limit, and yield strength ratio, while also possessing adequate plasticity and toughness—essentially, the best overall mechanical properties suited to the working conditions of a spring.

Fatigue Performance:

Fatigue failure is the primary failure mode of springs, and the most significant factors influencing the material's fatigue performance include its strength and toughness (high strength combined with sufficient ductility), as well as the material's surface condition—such as surface defects, decarburization, and residual stresses—and aspects like its microstructure, uniformity, and purity, all of which greatly affect the spring's fatigue life.

Bullet-resistant and deformation-resistant performance: (also known as anti-relaxation performance);

This refers to the spring's ability to resist deformation under long-term loading conditions at room temperature or operating temperatures, as permanent deformation of the spring during operation can lead to reduced load-bearing capacity—or even failure altogether.

Chemical properties:

Such as the material's corrosion resistance and high-temperature oxidation resistance.

Physical properties:

Such as the material's electrical and magnetic conductivity properties.

Requirements for the process performance of spring materials:

The material must exhibit sufficient ductility and excellent bending, torsion, and winding properties during both cold coiling and hot forming in the spring manufacturing process. For springs requiring heat treatment—particularly those manufactured through hot forming—thermal processing performance must be carefully considered. The material should possess adequate hardenability and minimal tendency toward surface decarburization.

2. Strengthening and Toughening Paths for Spring Materials:

Cold Work Hardening:

Plastic deformation of a material at room temperature leads to a continuous increase in its resistance to further deformation—specifically, after the material yields on the stress-strain curve, the stress keeps rising as strain continues to accumulate. From a microscopic perspective, this phenomenon arises from the multiplication of dislocations during lattice sliding within the crystal structure, which in turn elevates the dislocation density and hinders additional lattice movement, thereby enhancing the material's strength. This is known as strain hardening. Cold working is widely utilized in both spring materials and the manufacturing processes of springs—for instance, cold drawing of steel wires following lead-bath quenching, cold drawing and cold rolling of stainless steel materials, and even surface shot peening treatments applied during spring fabrication—all of which leverage the strain-hardening effect to improve material properties.

Heat Treatment Strengthening:

Strengthening materials through heat treatment is the most commonly used method in the manufacturing process of mechanical parts. In spring production, the primary heat treatment methods employed include the following:

Quenching and tempering treatment:

In addition to leveraging the strength and toughness of the tempered martensite structure obtained through quenching and tempering, refining the grain structure after quenching also contributes significantly to the strengthening effect.

Isothermal Quenching:

The bainitic microstructure obtained through isothermal quenching not only exhibits high strength but also boasts superior ductility and toughness, while components subjected to isothermal quenching experience minimal deformation.

Solid-solution aging treatment:

Utilizing the precipitation strengthening effect of the second phase within the material matrix.

Chemical Heat Treatment:

Also known as surface hardening treatment, this process enhances the performance of surface materials—such as improving spring characteristics and durability—by altering the surface's chemical composition (e.g., by diffusing elements like carbon or nitrogen into the surface).

Other reinforcement methods:

Surface shot peening treatment.

How to achieve optimal comprehensive mechanical properties of spring products through heat treatment processes:

The properties of metals and alloys reflect their internal microstructures. The diversity and complexity of these microstructures naturally lead to a wide range of mechanical properties, among others. The primary purpose of heat treatment for metal components is to induce complex, dynamic solid-phase transformation mechanisms, enabling the material to achieve the desired microstructure—and thereby endowing it with the specific properties required for various applications.

The most critical requirements for spring performance are ensuring the spring's fatigue life and its resistance to elastic relaxation. To enhance these properties in spring steel, it’s essential not only to improve purity but also to rigorously control the rolling process and heat treatment procedures, thereby boosting the steel's strength and toughness.

There are two main scenarios in the spring manufacturing process that require quenching and tempering: one involves directly performing residual-heat quenching after hot coiling, while the other entails re-heating followed by quenching and subsequent tempering. Notably, the austenitizing temperature and the process parameters for quenching and tempering significantly influence the overall mechanical properties of the final product.

The question of how to determine the heat treatment process method and its specific parameters based on product requirements can be addressed using the following approaches:

1. Typically, spring product drawings only specify hardness requirements. Generally, during the product trial manufacturing phase, the heat treatment process is determined based on the material grade, followed by small-batch trials. Once the hardness meets the specified criteria, mass production can commence. This approach is suitable only for products with relatively low performance requirements.

2. In addition to hardness requirements, technical specifications for spring product drawings also include other criteria such as fatigue life and resistance to elastic deformation. In these cases, the heat treatment process should prioritize ensuring that the final product meets all specified performance targets. If, after the heat treatment process has been finalized, the desired performance still cannot be achieved, additional measures—such as upgrading the material grade or implementing enhanced processes with supplementary strengthening techniques—can be adopted based on the actual situation. This approach is particularly applicable to medium-to-high-end spring products.

3. Spring products are classified as high-end items, and their product drawings specify particularly stringent requirements for fatigue life and impact-absorbing performance. Additionally, rigorous weight control and procedural standards are applied—from sample provision and small-batch prototyping to the transition into full-scale mass production. Under these circumstances, it is best to proceed as follows:

1. For technical reviews of products with pre-designed drawings, determine appropriate material selection and process plans, or specify materials and design processes based on product usage requirements and load conditions.

2. Conduct experimental verification of heat treatment process plans and parameters by combining them with sample prototyping, based on

The experimental results ultimately determined the heat treatment process.

3. Conduct performance and durability tests on the prototype samples, and determine the small-batch trial production process based on the test results.

4. Analyze and finalize the heat treatment process based on the installation tests or actual operation test results of the small-batch trial-produced products.

The heat treatment of spring products should be determined based on the material’s mechanical property requirements that align with the specific usage demands and load characteristics of the spring. While ensuring adequate strength and toughness after heat treatment remains a universal requirement, the optimal values for various mechanical properties differ depending on the distinct operating conditions and load specifications. For instance, springs designed for static loads but requiring high stability under sustained force need materials that exhibit high yield strength and excellent resistance to elastic deformation after heat treatment—though their ductility and toughness do not necessarily have to meet stringent criteria. In contrast, springs subjected to dynamic loads (including variable and impact loads) must prioritize superior toughness, while excessive emphasis on tensile strength and hardness should be avoided.

3. Related issues in spring heat treatment:

Spring Heat Treatment Hardness:

Most spring heat treatments employ quenching and tempering processes, with the specified requirements typically indicated as the acceptable hardness range after treatment. However, the primary goal of spring heat treatment is to achieve the desired performance characteristics and reliability—such as anti-relaxation properties and fatigue life—required for the spring's application. Therefore, determining whether a heat treatment process is suitable ultimately depends not just on hardness, but also on other material properties like strength and toughness after treatment.

The relationship between hardness and tensile strength:

When specifying the acceptable hardness range for spring heat treatment, we need to consider the relationship between hardness and tensile strength, as the material's tensile strength, R, is incorporated into the calculation formulas used during spring design. _m Therefore, it is necessary to perform conversions between hardness values and tensile strength. The national standard GB/T 1172-1999 provides {Conversion Values for Hardness and Strength of Ferrous Metals}, while the international standard ISO 18265-2003 offers {Metallic Materials – Conversion Values for Hardness}. Additionally, ISO/TR 10108-1989, titled {Steel – Conversion Between Hardness and Tensile Strength}, serves as a technical report that establishes conversion ranges and application guidelines specifically for Brinell and Vickers hardness scales to tensile strength.

Since there are similarities between indentation hardness tests such as Rockwell and Brinell hardness and uniaxial tensile tests, a certain relationship does exist between hardness values and strength values. However, these two types of tests remain distinct in nature, with differing stress-state softness coefficients. Moreover, both material hardness and tensile strength are influenced by multiple factors and governed by different underlying principles. Therefore, the conversion relationship between hardness and tensile strength inherently carries a degree of uncertainty.

Considering the above, for springs used in critical applications, the development of the heat treatment process should be based on the spring's service conditions—specifically, the nature and magnitude of the applied load—and must include verification tests against relevant mechanical performance criteria. Simply verifying hardness alone is insufficient. While hardness testing is straightforward and easy to perform, it can serve as an effective monitoring tool to ensure proper process execution or to detect abnormalities in equipment and process parameters. Since extracting samples directly from the spring itself for tensile testing is often challenging, an alternative approach is to use test bars cut from the same raw material as the product, which are then heat-treated alongside the production batch before being machined into specimens for mechanical property evaluation.

Hardenability:

Large-section springs often fail to meet hardness requirements during quenching. Therefore, it is crucial to carefully consider material selection and process planning in advance. Among these considerations, the most critical issue is the steel's hardenability. Hardenability is essential because if the steel lacks sufficient hardenability, parts undergoing quenching and tempering may end up with insufficient hardness. Meanwhile, for components subjected to normalizing treatment, even if the final hardness meets the specified requirements, the presence of partial non-martensitic structures formed during quenching can significantly degrade the material's mechanical properties.

The basic concept of hardenability:

Quenchability is an important heat-treatment property of steel and serves as a key criterion for material selection and the development of heat-treatment processes. First, it is necessary to define the standard (meaning) of "quenchability." For instance, one common standard defines quenchability as the hardness of a composite microstructure consisting of 50% martensite and 50% other transformation products—referred to as "semi-martensitic hardness." Alternatively, the distance J on the end-quench curve corresponding to the semi-martensitic region (known as "semi-quench distance") can also be used as a quantitative measure of quenchability. Of course, quenchability standards can also be established based on different percentages of martensite, such as 99% martensite or 90% martensite. The widespread use of 50% martensite, aside from its convenience in experimental determination, stems primarily from the fact that the foundational work for calculating quenchability based on chemical composition is rooted in Grossmann's research on the associative critical diameter D. _I The concept. And D _I It is defined as 50% martensite.

The hardenability of steel refers to its ability to achieve a certain depth of hardened layer (also known as the quench-hardened zone) during quenching—essentially, the capacity of the steel to transform supercooled austenite into martensite at a specific depth under given cooling conditions. It also reflects the stability of the steel's supercooled austenite. This property can be quantified by the critical cooling rate required for quenching, as defined by the steel's continuous cooling transformation (CCT) curve. The critical cooling rate is the threshold below which supercooled austenite fails to undergo the M transformation. _S Any minimum cooling rate required for transformations occurring above the Ms temperature (the temperature at which martensite begins to transform). Hardenability refers to an inherent property of each type of steel—indeed, the deeper the hardened layer achieved, the better the steel's hardenability.

The effective hardening depth of a real workpiece depends on many factors, such as the steel's hardenability, the workpiece size, and the cooling capacity of the quenching medium. For example, when quenching the same type of steel in the same medium, smaller parts will achieve a deeper hardened layer compared to larger ones. Similarly, for workpieces of the same steel and size, water quenching results in a deeper hardened layer than oil quenching.

Quenchability and hardenability are not the same concept—be sure to distinguish between them. Hardenability refers to a steel’s ability to achieve the highest possible hardness under ideal quenching conditions, which depends on the carbon content in the resulting martensite. In fact, it is primarily limited by the carbon content of the steel itself; therefore, low-carbon steels won’t attain very high hardness after quenching.

Factors affecting hardenability:

The hardenability of steel is determined by its critical cooling rate. The lower the critical cooling rate, the more stable the supercooled austenite becomes, and the better the steel's through-hardening capability will be. Therefore, any factor that influences the stability of austenite will also affect the steel's through-hardening properties. These factors include: the effect of carbon content, the influence of alloying elements, the impact of austenitizing temperature, and the presence of undissolved secondary phases in the steel.

The composition and smelting quality of the same type of steel will inevitably fluctuate within a certain range. Consequently, the hardenability curves provided in relevant manuals for a particular steel grade often do not appear as a single line, but rather as a range—referred to as the "hardenability band." Therefore, the hardenability of steel encompasses two key aspects: first, the material's inherent ability to harden, which primarily ensures consistent core hardness across parts of varying sizes, thereby meeting the strength and durability requirements of the components; and second, the width of the hardenability band itself, with tighter control over this band being crucial for minimizing distortions during heat treatment and maintaining superior heat-treatment quality. In China, the hardenability band typically measures 12 HRC, whereas leading international companies aim for a much narrower band, with widths no greater than 8 HRC.

The hardenability value of steel can be expressed as J (HRC / mm), where J represents end-hardenability, d indicates the distance from the water-cooled end, and HRC is the hardness value measured at that point. For example: a hardenability value of J (42 / 5) means the sample has a hardness of HRC 42 at a distance of 5 mm from the water-cooled end; while J (30~35 / 10) signifies a hardness range of 30–35 HRC at 10 mm from the same end. For steels with specific requirements on their hardenability values, the steel grade is marked with "H" at the end—for instance, 42CrMoH.

There are various other methods for evaluating the hardenability of steel, as steel’s hardenability is primarily determined by its chemical composition, leading to numerous calculation approaches based on the actual chemical makeup of the steel. For instance, the critical hardenability diameter D for the specific steel can be calculated this way. _I The calculation formula is as follows:

D.I(in) = 0.54C × (0.7Si + 1) × (0.3333Mn + 1) × (2.16Cr + 1) × (3Mo + 1) × (0.363Ni + 1) × (0.365Cu + 1) × (1.73V + 1)

According to ISO 683-14 standard, the recommended maximum dimensions (provisional) for the 10 steel grades specified are based on the following condition: The steels must be oil-quenched within the quenching temperature range indicated in the table, and the core hardness after quenching should reach either 54 HRC or 56 HRC.

Optimizing the Quenching Process:

In recent years, numerous studies have focused on optimizing the heat treatment processes for spring steels, with a particular emphasis on 60Si2Mn. Other steel grades containing elements such as Cr, Mo, V, and Ni have also been investigated. Many studies indicate that moderately increasing the quenching heating temperature of spring steel can enhance the material's ductility and toughness—or, alternatively, improve its strength properties while maintaining adequate levels of ductility and toughness. These strengthening and toughening effects are primarily attributed to:

a. The transformation of martensite morphology—specifically, increasing the quenching temperature leads to more lath martensite and less needle-like martensite in the quenched microstructure. Moreover, lath martensite exhibits superior overall mechanical properties after tempering, and the enhanced fracture toughness of the material further improves its fatigue performance.

b. Increasing the austenitizing temperature allows for more complete dissolution of carbides in the steel, resulting in a more uniform austenitic composition, which is beneficial for enhancing the steel's microstructure and mechanical properties.

It is generally believed that increasing the quenching heating temperature leads to the coarsening of austenite grains, thereby degrading the material's performance after heat treatment. However, in reality, for 60Si2Mn steel, the temperature at which austenite grain coarsening occurs exceeds 950°C; thus, moderately raising the quenching heating temperature will not cause significant grain growth. As for spring steels alloyed with elements such as vanadium and niobium, their carbides exhibit a strong precipitation-hardening effect. By elevating the austenitizing temperature, the solubility of these carbides can be increased, ultimately enhancing the overall precipitation-hardening performance.

Questions related to the tempering process: 。

The Influence of Alloy Elements on Tempering Transformation:

The decomposition process of martensite in the quenched microstructure of alloy steel is fundamentally similar to that of carbon steel, but the decomposition rate differs significantly. Alloying elements primarily impede the martensite decomposition process by influencing carbon diffusion—this effect varies depending on the strength of the interaction between the alloying elements and carbon.

Alloying elements—particularly strong carbide-forming elements such as Mo, W, V, Nb, and Ti—can effectively hinder the decomposition of martensite. In this case, only a portion of the carbon precipitates in the form of cementite, which minimizes the reduction in strength and hardness of the quenched steel, thereby enhancing its tempering stability. Interestingly, steels containing these strong carbide-forming elements can even exhibit a "secondary hardening" phenomenon at higher temperatures, during which fine, specialized carbides begin to precipitate.

Mechanical properties of quenched spring steel after medium- and high-temperature tempering:

In the quenched microstructure of spring steel, martensite is the primary phase, along with a small amount of retained austenite (and in steels with higher carbon content, a minor amount of undissolved carbides may also be present). In engineering applications, quenched martensite is rarely used directly for mechanical components; typically, these parts are heated again to A. ₁ Temperature treatments conducted below a certain point are commonly referred to as tempering. Traditionally, holding at temperatures between 20°C and 150°C is called aging treatment, while temperature ranges from 150°C to 250°C, around 350°C, and between 450°C and 550°C are respectively known as low-, medium-, and high-temperature tempering. For spring steel, tempering treatments are typically performed at medium or high temperatures.

To meet the diverse requirements of different products for the strength and toughness of material properties, it is essential to understand how the various mechanical properties of spring steel change as the tempering temperature increases after quenching and tempering treatments. This approach can provide valuable insights:

1. As the tempering temperature increases, the changes in mechanical properties follow the typical trend observed in medium- and high-carbon alloy steels after quenching and tempering: strength and hardness gradually decrease, while ductility (measured by elongation and reduction of area) shows a slight improvement.

2. Notably, the yield strength ratio (R) of the two steels is worth noting. _P0,2 /R _m With noticeable differences observed as temperature varies, the yield-to-tensile ratio of 55SiCrA steel remains nearly constant at around 0.91, whereas that of 50CrVA steel increases from below 0.92 to over 0.95 as the tempering temperature rises. The primary reason for this disparity lies in the alloying element vanadium, which is a strong carbide-forming element capable of creating fine VC particles. During tempering, the precipitation of these tiny VC particles induces a significant strengthening effect, enhancing the material's resistance to deformation—manifested as improved yield strength and a higher yield-to-tensile ratio. This unique performance characteristic makes 50CrVA steel particularly well-suited for spring products, especially those with stringent requirements for anti-relaxation properties. Moreover, since the dispersion-hardening effect originates from the dissolution of VC particles during the initial quenching and heating process, achieving complete dissolution is crucial for maximizing the subsequent precipitation-hardening benefits during tempering. Therefore, studies on spring steels like 60Si2CrVA suggest that raising the quenching temperature to approximately 910°C can yield optimal combinations of strength and toughness.

3. After quenching, the tempering of spring steel requires careful attention to the issue of temper embrittlement. Type I temper embrittlement occurs between 250°C and 400°C and is present in nearly all steels. Meanwhile, temper embrittlement appearing between 450°C and 600°C is referred to as Type II temper embrittlement. Elements such as Ni, Cr, Mn, Si, and C are alloy components that promote Type II temper embrittlement, whereas elements like Mo, W, V, and Ti act as inhibitors of this type of embrittlement. Rapid cooling immediately after tempering can effectively eliminate or mitigate Type II temper embrittlement. Moreover, for parts already exhibiting brittle characteristics, reheating followed by quick cooling can reverse the embrittlement, restoring their original toughness. This is why Type II temper embrittlement is also known as "reversible temper embrittlement." Currently, on our heat treatment production line, after tempering is complete, we immerse the springs directly into cooling water tanks—this step not only helps minimize temper embrittlement but also facilitates a swift transition to the next processing stage.

4. The scenarios described above illustrate that, for special steels like spring steel containing certain alloy elements, the microstructural transformations of quenched martensite during tempering are highly diverse and complex. Therefore, systematically studying the heat treatment processes for spring steel—typically employing orthogonal experimental methods—is the optimal approach to achieving the best possible improvements.