What heat treatment do leaf springs need?

Complete heat treatment is the core process that determines the elastic limit, tensile strength and fatigue life of leaf springs. Unprocessed spring steel will deform permanently and crack quickly under cyclic load. Standard qualified leaf springs made of 60Si2Mn or 50CrV4 alloy spring steel must go through a full heat treatment workflow: blank normalizing → forming hot bending → integral quenching → medium-temperature tempering → shot peening surface strengthening → stress relief tempering. Each procedure has fixed temperature parameters and functional purposes, detailed as follows.

1. Normalizing Treatment of Raw Steel Blanks

Before cutting and forming, steel strips are heated to 860–900°C and fully air-cooled.

Core function: Eliminate internal metal stress generated during steel rolling; refine coarse grain structure, reduce material hardness and improve cutting, punching and hot bending molding performance.

Without normalizing: The steel has uneven internal stress, resulting in warped, cracked edges during cutting, and inconsistent arch shape after forming.

2. Hot Bending Forming (Arch Setting)

Cut steel blanks are uniformly heated to 900–950°C, rapidly pressed on special molds to form the standard arch curvature, then air-cooled to room temperature to lock the preliminary spring camber.

Core function: Shape the designed free arch height of the leaf spring; achieve uniform thickness transition for parabolic springs.

Key note: Temperature must be strictly controlled. Excessively low temperature leads to incomplete forming and arch rebound after cooling; overheating causes coarse grains and reduced fatigue resistance.

3. Integral Quenching (Critical Hardening Step)

The formed leaf spring assembly is reheated to 850–880°C, then quickly immersed in quenching medium (water or polymer quenching liquid) for rapid cooling.

Core function: Transform internal metal microstructure into high-strength martensite, drastically lifting tensile strength and elastic limit. Qualified quenched leaf springs reach hardness HRC 48–55.

Risk of missing quenching: The steel remains soft, prone to permanent sagging under normal rated load with very short service life. Uneven quenching causes inconsistent hardness on different parts of the leaf spring, creating stress weak points.

4. Medium-Temperature Tempering (Indispensable Toughness Adjustment)

After quenching, hard brittle steel is heated to 480–520°C, held for sufficient heat preservation time, then slow air cooling. This is the most vital heat treatment to balance strength and toughness.

Core function: Eliminate huge internal tensile stress generated during quenching; convert brittle martensite into tough tempered troostite, avoiding spontaneous cracking after quenching. It stabilizes elastic performance and prevents arch deformation in long-term service.

Parameter control: Tempering temperature directly determines spring performance. Too low tempering retains brittleness and causes easy fracture under impact; over-tempering reduces hardness and elastic rigidity, triggering early sagging.

5. Shot Peening Surface Strengthening (Surface Compressive Stress Treatment)

Though categorized as surface mechanical strengthening instead of thermal heating, it is a mandatory matched process after quenching and tempering for heavy-duty leaf springs. High-speed steel shot bombards the spring surface uniformly from all directions.

Core function: Form a uniform compressive stress layer 0.1–0.3mm deep on the steel surface. This layer offsets tensile stress produced by bending load during driving, inhibits microcrack initiation and expansion, and extends fatigue life by over 30%.

Products skipping shot peening: Surface microcracks quickly expand under vibration load, and the spring is 50% more likely to break prematurely. Mining-grade 50CrV4 leaf springs require enhanced intensive shot peening.

6. Low-Temperature Stress Relief Tempering (Post-Peening Auxiliary Heat Treatment)

After shot peening, leaf springs are heated to 180–220°C with short heat preservation.

Core function: Remove tiny residual stress induced by shot impact; stabilize the compressive stress layer formed by peening to avoid stress layer failure during long-term alternating load.

Supplementary Heat Treatment Differentiation by Spring Grade

Standard highway 60Si2Mn multi-leaf / parabolic springs: Complete full-process heat treatment including normalizing, quenching, tempering and standard shot peening.

Heavy mining 50CrV4 reinforced leaf springs: Add extended heat preservation time for quenching and tempering, adopt double shot peening plus secondary stress relief tempering to improve impact fatigue resistance.

Inferior low-cost counterfeit springs: Skip tempering or shot peening, only simple hot bending and single quenching. Such products have poor toughness and fracture easily under mild overload.

The Consequence of Incomplete Heat Treatment

If any core heat treatment step is omitted or temperature parameters deviate severely:

Permanent arch sagging after short mileage;

Easy generation of surface fatigue cracks and sudden fracture;

Severe inter-leaf abrasion and rapid thinning of steel plates;

Poor impact resistance, unsuitable for mountain and mine road operation;

Large left-right chassis height difference after short service time.

In summary, qualified leaf springs rely on a full set of standardized heat treatment procedures: blank normalizing, hot arch forming, high-temperature quenching, medium-temperature tempering, shot peening strengthening and low-temperature stress relief tempering. Each process coordinates to balance strength, elasticity and toughness, realizing long-term stable load-bearing performance under complex vehicle working conditions.

1. APA 7th Edition

Zhang, L. (2026). Full heat treatment process specification and performance control of alloy steel leaf springs for commercial vehicles. Heat Treatment Technology of Automotive Components, 2(1), 169–176.

2. MLA 9th Edition

Zhang, Lei. "Full Heat Treatment Process Specification and Performance Control of Alloy Steel Leaf Springs for Commercial Vehicles." Heat Treatment Technology of Automotive Components, vol. 2, no. 1, 2026, pp. 169–176.

3. GBT 7714-2015

Zhang Lei. Specification and Performance Control of Complete Heat Treatment Process for Alloy Steel Plate Springs in Commercial Vehicles [J]. Heat Treatment Technology for Automotive Parts, 2026, 2 (1): 169-176