### Types of Heat Treatment for Springs
Spring materials can be made from **steel** and **non-ferrous metals**, but most are steel materials. The raw materials come in various forms such as **bars**, **wires**, **plates**, and **strips**. Some materials are supplied after undergoing processes like **hot-rolling**, **cold-rolling**, **cold-drawing**, followed by **annealing**, **quenching and tempering**, or **bainitic isothermal quenching**. After being made into springs, these materials undergo **final heat treatment**.
#### 3.1 Preparatory Heat Treatment
##### 3.1.1 Annealing
For various round steels and hard steel wires used in **hot-formed springs**, **soft annealing** (spheroidizing annealing) is necessary to reduce material hardness, improve machinability, and increase the capacity for cold plastic deformation. Spring steels should be annealed in a furnace with a **protective atmosphere** to prevent surface oxidation or decarburization. The annealed structure should be **spheroidized pearlite** with a hardness of about **180 HBW**.
##### 3.1.2 Normalizing
For certain springs, such as **automotive torsion bar springs**, hot processing can easily lead to uneven structures and properties, as well as large residual stresses. **Normalizing** can ensure a **uniform and fine grain structure** in spring steel, eliminate internal stresses, and facilitate subsequent forming processes.
#### 3.2 Final Heat Treatment
##### 3.2.1 Quenching
When **quenching** springs, it is essential to prevent surface **decarburization**, **overheating**, **burning**, and **grain boundary oxidation**. Springs must be heated in a furnace with a **protective atmosphere**, or a protective coating should be applied before heating. Depending on the type of steel, water or oil is used for cooling.
For example, **wave spring washers** with inner diameters of **φ12mm** and **φ16mm** used in automobiles were initially made from **65Mn** steel but were switched to **65#** and **70#** steel to prevent **hydrogen embrittlement**. The technical requirements include a hardness of **43-47 HRC**. The production process involves using an **SY-805-4 muffle-less continuous mesh belt furnace** at a heating temperature of **820°C ± 5°C**, with a belt speed of **35 min**, and controlling the carbon potential at **0.62%-0.65%**. The tempering temperature is **410°C ± 10°C**, with water cooling upon exit, resulting in a hardness of **43-45 HRC**.
**Operational Considerations**: When loading the furnace, ensure uniform loading to a height of **3-4 washers thick**. Avoid excessive stacking or scattered loading to prevent uneven heating or overheating. After quenching, springs should be quenched in **fast quenching oil** without using a stirrer to avoid excessive deformation. After tempering, springs should be quickly cooled in water to prevent **secondary temper brittleness**.
In production, **martensitic step quenching** or **bainitic isothermal quenching** is widely adopted. During bainitic isothermal quenching, a short holding time may be used to allow some austenite to remain, which, upon cooling to room temperature, transforms into martensite. This process yields a microstructure composed of both **bainite** and **martensite**, providing high strength and good toughness. This method requires a subsequent tempering process and is typically suitable for springs with small cross-sections.
##### 3.2.2 Tempering
Springs should be **tempered promptly** after quenching, with the delay between quenching and tempering not exceeding **4 hours**, to prevent quenching cracks and improve plasticity and toughness. Tempering should be performed with **uniform heating**. For spring steels prone to **secondary temper brittleness**, rapid cooling after tempering is necessary.
##### 3.2.3 Low-Temperature Annealing (Stress-Relief Tempering)
Materials such as **piano wire**, **cold-drawn hard steel wire**, **stainless steel spring wire**, or **phosphor bronze** are commonly used for **cold-formed springs**. Quenched and tempered wire, **strain-hardened wire**, and heat-treated steel strips also fall into this category. These materials generate high internal stresses during forming, so **low-temperature annealing** (stress-relief tempering) is required to remove the stresses caused by forming.
The annealing temperature for stress relief is generally kept below the **recrystallization temperature**. After holding for an appropriate time, the springs are air-cooled. This process not only partially relieves internal stresses but also **stabilizes the shape and dimensions** and improves mechanical properties, such as increasing hardness (by **2-3 HRC**), tensile strength, yield-to-tensile ratio, **elastic limit**, fatigue strength, and stress-relaxation resistance.
##### 3.2.4 Precipitation Hardening
**Precipitation hardening** involves heating alloys above the **phase transition point** to form a supersaturated solid solution of certain elements, followed by rapid cooling. The springs and wires are then heated to an appropriate temperature above the solubility curve for that alloy, where they are held to allow fine **hard-phase particles** to precipitate uniformly throughout the supersaturated solid solution. This aging treatment improves the **strength** and **toughness** of the material, imparting the desired elasticity properties.
Common precipitation hardening materials used in spring manufacturing include **07Cr17Ni7Al (0Cr17Ni7Al)**, **Be-Cu alloy**, **Fe-Ni based high-elasticity alloys**, **Inconel X-750 (Ni73Cr15Fe7NbTi)**, and **Inconel 718**.
##### 3.2.5 Strain-Induced Heat Treatment
For **60Si2Mn** and **55SiMnB** steel **leaf springs**, **high-temperature strain-induced heat treatment** can enhance their strength, toughness, and fatigue life. Additionally, it improves the production efficiency of leaf springs, reduces labor intensity, and lowers manufacturing costs. In automotive leaf spring production, the leaf springs are **bent using a press** in a heated state, then quenched in **oil**, followed by medium-temperature tempering.
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These heat treatments are critical in ensuring that springs meet the required mechanical properties such as **strength**, **toughness**, **fatigue resistance**, and **dimensional stability**.