Mining equipment bolts bear long-term alternating impact loads. Single quenched & tempered temper cannot completely release residual stress from cold heading and rolling, leading to fatigue cracks at thread root after several months of service. Four-stage closed-loop tempering craft releases stress step by step and can cut failure ratio after 1 million fatigue cycles by more than 70%. This article unifies constant temperature and holding time standards of four-stage heat treatment, and compares fatigue performance difference of finished goods between single-stage and multi-stage temper.
| Heat Treatment Scheme | Temper Flow | Finished Hardness HRC | Failure Ratio After 1 Million Fatigue Cycles |
| Conventional Single-stage Quench & Temper | Quench + One-time Temper At 570℃ | 40~43 | 0.28% |
| Four-stage Closed-loop Optimized Temper | Quench + 570℃ Quench & Temper + 210℃ Pre-plating Stress Relief + 230℃ Post-plating Hydrogen Removal | 39~41 | 0.08% |
1. Vacuum oil quenching at 880℃: Isolate air oxidation decarburization to guarantee uniform overall substrate structure; 2. Primary quenched & tempered temper at 570℃: Reach basic strength of grade 12.9 and eliminate internal quenching stress; 3. Low-temperature stress relief at 210℃ before electroplating: Release concentrated residual stress of cold heading threads; 4. Constant temperature hydrogen removal at 230℃ after zinc / nickel plating: Disperse hydrogen atoms penetrated during electroplating to prevent delayed hydrogen embrittlement fracture.
1. Fatigue crack at thread root: Missing pre-plating stress relief temper leading to stress concentration; Rectify: Mandatorily add secondary stress relief procedure at 210℃. 2. Delayed fracture after storage (hydrogen embrittlement): No post-plating hydrogen removal baking; Rectify: All high-strength parts must be held at 230℃ for 3 hours after plating. 3. Excessively high hardness & high brittleness leading to impact fracture: Primary temper temperature lower than 560℃; Rectify: Stabilize quenched & tempered temper within 560~580℃ range. 4. Transverse fatigue fracture on shank: Insufficient temper holding time with partial residual stress; Rectify: Extend holding time by 0.5min/mm according to bolt diameter.