Heat treating steel fasteners (screws and bolts) is essential for achieving the required strength, toughness, and wear resistance. Different hardening processes yield different hardness levels, case depths, and costs. Heat treatments like quenching and tempering, case hardening (carburizing), nitriding, and induction hardening each produce distinct surface and core properties. Choosing the right method depends on the needed hardness distribution, fatigue life, and budget.

Each method offers trade-offs between hardness, depth of hardened layer, distortion, and cost. For example, quench-and-temper (through hardening) gives uniform strength and is very cost-effective for general-purpose fasteners. Case hardening (carburizing) adds carbon at the surface to create a hard skin while retaining a ductile core, ideal for wear-prone threads and gear teeth. Nitriding (diffusing nitrogen) produces the highest surface hardness and superior fatigue/corrosion resistance but requires special equipment and longer processing time. Induction hardening uses an electromagnetic coil to heat only the surface before quenching, allowing very fast cycle times and precise hardening of sections or small parts.

Choosing the Right Method:
– Quench & Temper (Through Hardening): Best for general high-strength screws, bolts, and shafts when deep, uniform hardness and toughness are needed at low cost. Typical final hardness is around 50–60 HRC throughout the part.
– Case Hardening (Carburizing/Carbonitriding): Use when a hard, wear-resistant surface is needed on low-carbon steel parts. This is common for long screws, bolts, or gears where the outer thread or tooth must resist wear but the core must absorb shock. Surface hardness can reach ~60 HRC, with a tough core. The process is moderately expensive due to furnace soak time.
– Nitriding: Ideal for small high-precision screws and components needing maximum surface hardness and fatigue strength (often in corrosive environments). Nitrided parts can achieve very high surface hardness (~60–65+ HRC) and compressive residual stress, but the process is slow and costly. It’s suited to alloy steels and parts that benefit from a uniform, distortion-free case.
– Induction Hardening: Well-suited for selective hardening of long screws, shafts, and ball screws in high-volume production. Induction hardening yields a hard surface layer (~60 HRC) with minimal distortion and relatively deep case depth, and is faster (and often cheaper per part) than furnace case treatments. It’s a good choice when you need a wear-resistant surface quickly while maintaining corrosion resistance (no added carbon).

Each heat treatment method has its niche: use quenching & tempering for cost-effective bulk hardening; case hardening for a hard surface with a ductile core; nitriding for the hardest, most fatigue-resistant surface layers; and induction hardening for fast, localized surface treatment. Selecting the appropriate process depends on the screw’s required hardness, case depth, fatigue life, distortion tolerance, and budget.