Introduction to Science: Understanding Screw Heat Treatment: A Key Step in Determining Strength and Toughness

As core components of mechanical connections, screw performance directly determines the reliability and safety of equipment. Heat treatment is a critical process that modifies the internal structure of screws by controlling the heating, insulation, and cooling processes to achieve the desired mechanical properties (such as strength, hardness, and toughness). Screws made of different materials (such as carbon steel, alloy steel, and stainless steel) require tailored heat treatment solutions to meet the requirements of diverse applications (such as automotive, construction, and aerospace).

The core purpose of screw heat treatment

Screws must withstand loads such as tension, shear, and impact during operation, and some must also withstand harsh environments such as corrosion and high temperatures. The core goal of heat treatment is to strike a balance between strength and toughness, which can be categorized into three main categories:

Performance Enhancement (the most important objective): By modifying the internal structure (such as forming martensite or sorbite), the tensile strength, yield strength, and hardness of the screw are increased, preventing plastic deformation or fracture under load. (Typical applications include automotive engine block screws and bridge connection screws, which must withstand high loads without deformation.)

Relieve Internal Stress: After cold heading (forming) and machining, residual stress remains within the screw, which can easily lead to cracking or dimensional deformation during subsequent use. Heat treatment, through processes such as low-temperature tempering and stress relief annealing, can release these internal stresses and ensure dimensional stability. (Typical use case: Micro screws used in precision instruments require extremely high dimensional accuracy (e.g., tolerances of ±0.01mm).)

Improving Machinability: Some high-hardness materials (such as high-carbon steel) are difficult to machine directly. Annealing can reduce hardness and increase plasticity, facilitating cold heading or threading. Quenching and tempering can then be used to increase strength. (Typical use case: 45# steel screws are annealed before forming (to reduce hardness to HB180-220), followed by quenching and tempering after machining (to increase hardness to HRC35-40).)

Common screw materials and corresponding heat treatment processes

The choice of screw material determines the heat treatment route. The differences in composition (such as carbon content and alloying elements) between different materials lead to completely different phase transformation characteristics and performance requirements. The following are process combinations for three mainstream materials:

Low-carbon steel Q235, 10# steel: Core heat treatment process (carburizing + quenching + low-temperature tempering)

Medium-carbon steel 45# steel, 35# steel: Through-hardening + medium-temperature tempering

Alloy structural steel 40Cr, 35CrMo: Quenching and tempering (quenching + high-temperature tempering)

Martensitic stainless steel 410, 420: Quenching + low-temperature tempering

Key process links of screw heat treatment

Screw heat treatment requires strict control of the three-stage parameters of "heating - holding - cooling" to avoid defects such as insufficient hardness, cracking, and deformation. The following is a detailed analysis of the core process:

Pretreatment: Annealing/Normalizing (preparing for subsequent processing or final heat treatment)

Annealing: Slowly heat the screw to 30-50°C above Ac3 (hypoeutectoid steel) or Ac1 (hypereutectoid steel), hold for a period of time, and then slowly cool in the furnace (cooling rate ≤ 50°C/h).

Purpose: Reduce hardness (e.g., 45# steel hardness ≤ HB229 after annealing), relieve processing stresses, and refine the grain size in preparation for cold heading or quenching.

Normalizing: Heating to a temperature similar to annealing, but holding followed by cooling in air (cooling rate faster than annealing).

Purpose: Produce a finer pearlite structure with a slightly higher hardness than annealing (45# steel hardness HB170-230 after normalizing). Suitable for non-critical screws with certain strength requirements.

Strengthening Treatment: Quenching + Tempering (Determines the Screw's Final Mechanical Properties)

(Quenching) Achieves high hardness, but also brittleness: The screw is heated to the "austenitizing temperature" (e.g., 840-860°C for 45# steel, 830-850°C for 40Cr steel), held at this temperature to allow the microstructure to completely transform into austenite. Rapid cooling (e.g., water or oil cooling) allows the austenite to transform into martensite, significantly increasing the hardness.

(Tempering) Balancing hardness and toughness (the core "tuning" step): The quenched screw is reheated to "sub-Ac1 temperature" (no higher than 727°C to avoid austenitization), held at this temperature, and then cooled to partially decompose the martensite into tempered martensite, troostite, and troostite, reducing brittleness while retaining a certain degree of hardness.

Surface Hardening: Carburizing/Nitriding (for high surface hardness requirements)

For low-carbon steel screws (such as 10# steel), due to their low carbon content (≤0.15%), full quenching cannot achieve high hardness. Surface carburizing is required to increase the surface hardness while retaining the toughness of the core.

Carburizing Process: The screw is placed in a carburizing furnace (containing a carburizing agent such as methane or propane) at 900-950°C for 2-6 hours to raise the surface carbon content to 0.8%-1.2%. The screw is then quenched and tempered at low temperature.

Common defects and prevention of screw heat treatment

During the heat treatment process, improper parameter control or operational errors will cause the screws to be scrapped. Common defects and preventive measures are as follows:

Insufficient Hardness

Causes: 1. Quenching temperature too low; 2. Insufficient holding time; 3. Slow cooling rate
Preventative Measures: 1. Set quenching temperature according to material specifications; 2. Ensure sufficient holding time; 3. Use water quenching for low-carbon steel and oil quenching for alloy steel

Quenching Cracking

Causes: 1. Excessive heating rate (large internal and external temperature difference); 2. Excessive cooling rate; 3. Sharp corners/cracks in the screw

Preventative Measures: 1. Slow heating (staged heating); 2. Use oil quenching or austempering for alloy steel; 3. Remove sharp corners during processing and inspect for surface defects in advance

Dimensional Deformation

Causes: 1. Uneven heating/cooling; 2. Asymmetrical screw shape; 3. Insufficient tempering

Preventative Measures: 1. Use a uniform heating furnace and rotate the screw during cooling; 2. Optimize screw design (reduce wall thickness variations); 3. Temper promptly after quenching.

Oxidation and decarburization

Cause: Excessive air in the heating furnace, leading to surface oxidation or carbon loss.

Preventative measures: 1. Use a protective atmosphere furnace (nitrogen/hydrogen); 2. Apply anti-oxidation coating to the screw surface before heating.