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How Are Springs Heat Treated? Tempering and Stress Relief Explained

Springs are heat treated through two distinct processes: stress relief, performed after cold coiling or forming at temperatures between 260°C and 430°C (500°F to 800°F) to remove residual stresses, and tempering, performed after hardening (quenching) at temperatures between 315°C and 540°C (600°F to 1000°F) to restore ductility and set final mechanical properties. The specific cycle depends on the alloy, required hardness, and whether the spring is pre-hardened or oil-tempered wire. For most cold-formed springs made from hard-drawn or oil-tempered wire, only low-temperature stress relief is required; for hot-wound or high-performance springs, full quench and tempering is mandatory.

What Is the Difference Between Tempering and Stress Relief for Springs?

Stress relief is a low-temperature heat treatment (260°C to 430°C) applied to cold-formed springs to remove internal stresses introduced during coiling, bending, or grinding. It does not significantly change the microstructure or hardness of the wire; its purpose is purely to stabilize the geometry and prevent distortion or premature fracture. Tempering, by contrast, is a higher-temperature process applied after austenitizing and quenching, where the spring steel is heated to 315°C to 540°C to transform brittle martensite into tempered martensite, trading some hardness for toughness and fatigue resistance.

For springs made from pre-tempered wire (e.g., oil-tempered chrome silicon), stress relief is the only thermal operation needed, performed at a temperature 50°C to 100°C below the wire's original tempering temperature to avoid softening. For springs made from annealed wire (e.g., 5160 or 1075), full heat treatment is required: austenitize at 830°C to 870°C, quench in oil, then temper. The choice is dictated by the wire specification and the spring's service load.

What Temperatures and Times Are Used for Spring Heat Treatment?

Time and temperature are tightly coupled; the goal is to achieve the target hardness (typically 42 to 52 HRC for most compression springs) without decarburization or grain growth. Stress relief cycles run for 20 to 60 minutes, while tempering cycles run for 1 to 2 hours. The table below lists standard parameters for common spring materials.

For stress relief, the rule is 1 hour per 25 mm of wire diameter, but never less than 20 minutes. For tempering, soak time must be sufficient to equalize temperature through the cross-section; a 10 mm wire needs at least 45 minutes at tempering temperature.

How Does Heat Treatment Affect Spring Fatigue Life and Load Capacity?

Fatigue life is directly proportional to the residual stress state and surface hardness. Proper stress relief increases fatigue life by 20% to 40% compared to untreated springs because it eliminates micro-crack initiation sites caused by residual tensile stresses on the surface. Tempering at the correct temperature maximizes the endurance limit; for chrome-silicon wire, tempering at 425°C yields a fatigue limit of approximately 700 MPa at 10^7 cycles, versus only 450 MPa for untempered martensite.

Load capacity (spring rate) is not changed by heat treatment because it depends on geometry and shear modulus, not hardness. However, the maximum safe deflection and load before permanent set are increased by 15% to 25% after tempering, because the yield strength in torsion rises from about 60% of tensile strength in as-quenched condition to 75% to 85% after proper tempering. Over-tempering (above 540°C) reduces hardness below 40 HRC and should be avoided for dynamic applications.

Which Spring Materials Require Full Quench and Tempering?

Full quench and tempering is required for hot-wound springs made from annealed carbon or alloy steels, including AISI 1065, 1075, 1095, 5160, 6150, and 9260. These materials arrive in a soft, spheroidized-annealed condition for formability; after coiling at 900°C to 1000°C, they must be austenitized, quenched in oil (or polymer quenchant), and tempered to achieve the desired strength. Springs over 12 mm in wire diameter are almost always hot-wound and fully heat treated because cold forming is impractical.

Pre-hardened wires, such as oil-tempered chrome-vanadium (ASTM A231), oil-tempered chrome-silicon (ASTM A401), and hard-drawn MB (ASTM A227), do not require austenitizing. They are cold-coiled and then only stress-relieved. Stainless steel springs (302, 316, 17-7 PH) are also cold-coiled and stress-relieved; 17-7 PH may require precipitation hardening at 480°C for 1 hour, which is a third distinct process.

Why Is Decarburization Control Critical During Spring Heat Treatment?

Decarburization is the loss of carbon from the surface layer of the spring steel, which reduces surface hardness and creates a soft skin that dramatically lowers fatigue strength. A decarburized layer of just 0.1 mm on a 5 mm wire can reduce fatigue life by 50% or more. During austenitizing at 830°C to 870°C, the furnace atmosphere must be controlled to a carbon potential equal to the steel's carbon content; this is achieved using endothermic gas with dew point control (0°C to 5°C) or vacuum furnaces for precision springs.

For stress relief at 260°C to 430°C, decarburization is not a risk because temperatures are too low for carbon diffusion. However, if stress relief is performed in a non-inert atmosphere and the spring has been previously quenched, there is a risk of blue-brittleness if temperatures exceed 430°C. The acceptable decarburization depth per ASTM A125 is 0.05 mm maximum for springs under 10 mm wire diameter, and verification is done by microhardness testing or metallographic inspection.

How Do You Verify Heat Treatment Quality for Springs?

Verification involves three tests: hardness testing on the surface and cross-section (Rockwell C scale), microstructural examination for tempered martensite (no untempered martensite allowed), and a load-deflection test at room temperature. Hardness should be measured at least 10 mm from the spring end to avoid end-coil effects; typical acceptance criteria are within ±3 HRC of the specified value. For critical springs, a proof load test at 110% of maximum design load is performed, and the spring must show no permanent set after 24 hours.

Additionally, magnetic particle inspection or eddy current testing is performed on 100% of springs used in automotive or aerospace applications to detect surface cracks from quenching. A salt bath or vacuum process is preferred over atmosphere furnaces for springs under 3 mm wire diameter because rapid heating in a salt bath minimizes distortion. Documentation should include the actual time-temperature chart and the furnace calibration certificate.

Can Springs Be Heat Treated After Shot Peening?

Shot peening is always performed after heat treatment, never before. The sequence is: heat treatment (stress relief or quench-temper) → shot peening (if specified) → optional low-temperature stress relief at 150°C to 200°C for 30 minutes to remove peening-induced micro-stresses. Some specifications require a second tempering step after shot peening when the peening intensity is high (Almen intensity above 0.020 A), but this is rare and must be approved by the design engineer.

The reason is that shot peening introduces compressive residual stresses of 600 to 1000 MPa on the surface, which are beneficial for fatigue. If the spring were heat treated after peening, these stresses would be annealed out and the peening effect lost. For springs with a pre-set operation (compressing to solid height), the pre-set is also done after heat treatment, at room temperature.

What Are the Common Heat Treatment Defects and How Are They Prevented?

The three most common defects are quench cracking, soft spots, and excessive distortion. Quench cracking occurs when the cooling rate is too fast or the wire diameter exceeds the hardenability limit; prevention requires using oil with the correct quench severity (GMG 204) and delaying quenching until the spring reaches 650°C to 700°C during cooling. Soft spots are caused by non-uniform heating or decarburization; prevention requires controlled furnace atmosphere and sufficient soak time.

Distortion is minimized by using fixtures or mandrels during heat treatment, especially for long compression springs (length-to-diameter ratio above 4). For springs with a free length tolerance of ±1%, heat treatment must be followed by a hot-setting operation at 25°C below the tempering temperature for 30 minutes under load. If distortion exceeds 0.5% of free length, straightening is required, but this should be avoided because it introduces residual stresses.

FAQ

How Long Does Spring Heat Treatment Take?

A typical stress relief cycle takes 30 to 60 minutes total, including furnace ramp-up, soak, and cooling. Full quench and tempering takes 2 to 4 hours per batch, including austenitizing (30 minutes), quenching (5 minutes), and tempering (60 to 90 minutes). For high-volume production, continuous belt furnaces process springs in 20 to 40 minutes per part.

What Is the Maximum Hardness Achievable After Spring Heat Treatment?

For carbon steel springs, the maximum practical hardness is 55 HRC after quenching, but tempering is always applied to reduce it to 42 to 52 HRC for toughness. Chrome-silicon alloys can reach 54 HRC, while stainless 302 typically peaks at 42 HRC. Hardness above 55 HRC is avoided because it causes brittle fracture under cyclic loading.

Can Stainless Steel Springs Be Heat Treated Like Carbon Steel?

No. Austenitic stainless (302, 316) cannot be hardened by quench and tempering; they are only stress-relieved at 315°C to 370°C. Precipitation-hardening grades (17-7 PH) require a special age-hardening cycle at 480°C for 1 hour. Martensitic stainless (410, 420) can be quenched and tempered but is rarely used for springs due to poor fatigue properties.

What Is the Cost Difference Between Stress Relief and Full Heat Treatment?

Stress relief costs approximately 0.05 to 0.15 USD per kilogram of spring weight, depending on batch size. Full quench and tempering costs 0.20 to 0.40 USD per kilogram due to the extra austenitizing step, quenchant, and tighter atmosphere control. For a typical 50 g compression spring, this adds 0.01 to 0.02 USD per piece.

When Should a Spring Be Hot-Wound Instead of Cold-Coiled?

Hot-winding is used when the wire diameter exceeds 12 mm, when the spring index (D/d) is below 4, or when the material is annealed alloy steel. The rule of thumb is that if the required forming force exceeds 50% of the wire's ultimate tensile strength, hot-winding is necessary. Hot-wound springs always require full quench and tempering.

How Does Heat Treatment Affect Spring Dimensions?

Stress relief causes a slight dimensional change of 0.1% to 0.3% in free length and outside diameter due to residual stress relaxation. To compensate, springs are often coiled oversize by 0.5% to 1% and then adjusted after heat treatment. Full quench and tempering can cause distortion of 1% to 2% in long springs, requiring straightening or hot-setting.

Is Vacuum Heat Treatment Better Than Atmosphere Furnaces for Springs?

Vacuum heat treatment provides zero decarburization and cleaner surfaces, making it superior for springs above 40 HRC or for aerospace applications. However, it costs 3 to 5 times more per batch than atmosphere furnaces. For standard industrial springs, atmosphere furnaces with dew point control are sufficient and more economical.

For precision spring heat treatment with full traceability and lot control, BQUQ offers stress relief and tempering services for springs from 0.1 mm to 30 mm wire diameter. Our furnaces are calibrated to AMS 2750E standards, and we provide microhardness and fatigue test reports with every batch. We can provide a heat treatment quote and production lead time within 12 hours of receiving your spring drawing or sample. Contact us at sc@bquq.com, WhatsApp +86 13713157787, or visit www.bquq.com.

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