Heat Treatment of Fasteners: Getting It Right for Every Grade
Heat treatment is a critical process in fastener manufacturing that directly affects mechanical properties like strength, hardness, and toughness. When done correctly, it ensures compliance with specifications and long-term reliability. But if mishandled, it can introduce serious flaws—often undetectable until failure occurs.
This blog explores the different heat treatment (HT) cycles for fastener materials, criteria for vendor selection as per API 20H, control of critical process parameters, traceability practices, and common mistakes to avoid.
Why Heat Treatment Matters
Fasteners are often used in high-stress, high-temperature, or corrosive environments. The heat treatment process customizes the microstructure of metals to meet specific mechanical requirements such as:
- Tensile strength
- Yield strength
- Hardness
- Toughness
- Fatigue resistance
- Each material grade demands a tailored HT cycle, including:
Control of Critical Process Parameters
Controlling the following parameters is essential to avoid defects and ensure repeatability:
- Furnace Temperature: Calibrated and regularly verified; deviations can lead to over- or under-heating.
- Soak Time: Based on part size and material; insufficient soak can result in incomplete transformation.
- Cooling Rate: Often critical for phases like martensite formation or avoiding sensitization.
- Atmosphere Control: Oxygen, nitrogen, or vacuum levels must be maintained to prevent oxidation or decarburization.
- Load Placement: Uniform spacing ensures even heating.
Each batch should be supported by:
- Furnace calibration certificates (NABL approved)
- Process charts with time vs temperature data
- Batch traceability linked to lot numbers
What Can Go Wrong? Common Mistakes and Root Causes
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Mistake
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Possible Root Cause
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Impact
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|---|---|---|
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Mixed-up grades in same batch
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Poor batch control
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Wrong mechanical properties / compliance
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Incorrect soak time
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Human error or lack of HT cycle knowledge
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Incomplete transformation / brittleness
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Delayed quenching (for Duplex)
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Poor planning / long transfer time
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Precipitation of intermetallic phases
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Soft zone in part
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Improper load placement in furnace
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Inconsistent performance / localized failure
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Criteria for Selecting a Heat Treatment Vendor (API 20H)
API 20H outlines expectations for heat treatment subcontractors handling critical fasteners. Key criteria include:
- NADCAP or ISO 17025 certified facility (as applicable)
- Documented HT procedures with revision control
- Qualified operators with records
- Batch traceability linked to mill test reports and inspection data
- Periodic audits and sample verification
Traceability & Handling Practices
Traceability should be maintained throughout the HT cycle:
- Unique batch identifiers
- Tags with material grade, lot, and heat treatment cycle
- Separate trays for each order with tamper-proof labeling
Transfer time from furnace to quench tank (especially for Duplex/Super Duplex) should be within seconds to prevent intermetallic phase formation.
Parts should be handled using non-contaminating tools to avoid surface pick-up or damage. Post-treatment inspection should include:
- Hardness testing
- Microstructure verification (grain size, phase)
- Surface evaluation for oxidation or decarburization
At Industrial Engineering Works (IEW)
We work only with pre-qualified HT vendors audited as per our internal procedures and API 20H requirements. Our process control and traceability ensure every part leaving our plant meets mechanical, metallurgical, and customer-specific standards.
Whether it’s a batch of B7 studs or Duplex flange bolts, we treat every fastener like a mission-critical component—because in most cases, it is.
Need fasteners with certified heat treatment and full traceability?
Let’s talk.