Final Heat Treatment Process for Fasteners

I. Common Heat Treatment Process Characteristics and Influencing Factors

The heat treatment process performance generally includes hardenability, hardening capacity, sensitivity to overheating and overburning, deformation and cracking tendency, as well as oxidation and decarburization sensitivity. The main factors influencing heat treatment process performance include part structure, technical requirements, material characteristics, raw material quality, and pre-treatment condition.

1. Hardenability and Hardening Capacity

① Hardenability
Hardenability refers to the ability of steel to be hardened by quenching — that is, the depth of the hardened layer. The standard measure is the depth at which 50% martensite (by volume) is achieved. However, standards such as GB/T3098.1 and GB/T3098.23 require that materials used for 8.8 (≥M16), 9.8, 10.9, and 12.9 grade fasteners must achieve 90% martensite at the core of the thread section after quenching and before tempering.

② Hardening Capacity
This refers to the maximum hardness achieved after quenching. It largely depends on the carbon content. Bolts of different sizes may not achieve the same hardness due to size effects — this is crucial and must be monitored carefully.

2. Overheating and Overburning Sensitivity

① Overheating Sensitivity
This refers to how easily the grain structure coarsens during heating. Coarse-grain steels are more sensitive, and high manganese content in fine-grain steels also increases sensitivity. Alloying elements that form carbides help prevent grain growth, reducing sensitivity.

② Overburning Sensitivity
Overburning refers to the tendency of grain boundary melting or oxidation during heating. Steels that are sensitive to overheating are often also prone to overburning. Low-melting impurities at grain boundaries further increase this risk.

3. Deformation and Cracking Sensitivity

① Deformation Sensitivity
Refers to how easily the fastener deforms during heat treatment. Complex shapes, high length-to-diameter ratios, or thin-walled areas increase risk. Steel type also affects deformation — in general:

• Water-quenched steel > Oil-quenched steel > Air-cooled steel
  High alloy steels resist deformation more than carbon or low-alloy steels.

② Cracking Sensitivity
Cracking during quenching can result from sudden size changes or sharp corners (e.g., R < 0.6 mm or angle < 90°), which create stress concentrations. Material brittleness and impurity levels also play a role:

• Water-quenched steel > Oil-quenched steel > Air-cooled steel
  Lower fracture strength and more impurities increase the risk of quenching cracks.

4. Oxidation and Decarburization Sensitivity

① Oxidation Sensitivity
Refers to the tendency of surface iron or alloy elements to oxidize during heating. Higher temperatures, lower carbon content, and active alloy elements increase oxidation risk.

② Decarburization Sensitivity
This is the tendency of surface carbon to burn off during heating. Elements like Si, Co, and Al promote graphite formation and increase carbon activity, raising the risk of decarburization.
For example:

• Molybdenum carbides are less stable than tungsten carbides, so Mo-alloyed steels are more prone to decarburization than W-alloyed steels.

Note: Oxidation and decarburization often occur together. Factors that promote decarburization also tend to promote oxidation.

II. Principles for Developing Heat Treatment Processes

For high-strength bolts, the goal is to achieve tempered sorbite or tempered bainite + sorbite to ensure comprehensive mechanical performance. This requires the core to form martensite after quenching, which depends on hardenability.

Since chemical composition varies between steel batches and suppliers, especially for cold-heading steels, adjustments are needed. The entire cross-section must be able to form 90% martensite to avoid quench cracking, which occurs when maximum tensile stress forms near the surface due to combined structural and thermal stresses.

Critical diameters for quench cracking:

• Water quenching: 8–12 mm

• Oil quenching: 20–39 mm

Quenching is a crucial step in the bolt tempering process. To ensure strength and stress remain within acceptable ranges, while increasing hardness, five factors must be controlled during tempering:

1. Material differences

2. Furnace type

3. Forging method (hot heading vs. cold heading)

4. Thread type (full thread vs. partial thread)

   • Full thread bolts have smaller effective cross-sectional area and lower tensile strength. A small tempering deviation (5–10℃) affects properties.

5. Quenching medium (water vs. oil)

III. Key Points in Mesh Belt Furnace Operation

Mesh belt furnaces are ideal for heat-treating small- to medium-sized high-strength fasteners due to their high automation, consistent quality, and ability to process fine-threaded parts.

1. Characteristics of Mesh Belt Furnaces

a. Intelligent Control

• Multi-parameter automation

• Speed, temperature, and carbon potential control

• Data storage for 10+ years

b. High Quality

• Temperature fluctuation: ±5℃

• Uniformity within the furnace: ≤10℃

• Carbon potential uniformity: ±0.05%C

• Equipped with: oxygen probes, decarburization air pumps, carbon controllers, and gas regulators

• Atmosphere: Methanol + Propane (or natural gas/toluene)

• Carbon potential: 0.33%–0.45%

2. Operation Essentials

a. Cleaning is critical
Bolts with residual oil and gas can significantly affect furnace atmosphere, causing:

• High CH4 and CO2

• Low CO

• Excessive carbon black formation

b. Loading uniformity

• Temperature fluctuation in heating zones should not exceed 5–10℃

• Avoid temperature drops >40–50℃ in zone 1 during feeding