Stock Allowances for Post-Hardening Finish Machining: How Much to Leave
When a machined part is heat treated, it distorts, scales, and grows slightly as the steel transforms during heating and quench. UTEC Industrial provides precision CNC machining services for large and oversized industrial components in the Pacific Northwest, with in-house heat treatment and induction hardening integrated into the machining workflow. If machined to final dimensions before heat treatment, this distortion has no correction path. The solution is a finish stock allowance on every precision surface — material that absorbs distortion and is removed in post-hardening finish machining to restore final tolerance. This article covers the sources of heat treatment distortion, recommended stock allowances by part size and hardening process, and the post-hardening finish machining strategy.
What causes distortion and dimensional change during heat treatment?
Heat treatment distortion originates from three distinct mechanisms, each contributing a different type and magnitude of dimensional change. Volumetric phase transformation: when steel transforms from austenite (face-centered cubic crystal structure, formed during heating above the critical temperature) to martensite (body-centered tetragonal, formed during rapid quench), the crystal structure change produces a volume increase of approximately 0.001–0.004 inch/inch (0.1–0.4%) depending on carbon content. A 4-inch diameter shaft will grow approximately 0.004–0.016 inches in diameter during a through-hardening quench — enough to push a pre-machined bore from undersized to oversized and a pre-machined OD from on-size to oversized. Thermal gradients during quench: when the part is quenched, the outer surface cools and transforms to martensite before the core. The surface contracts as it quenches while the core is still hot and austenitic — producing compressive stress at the surface and tensile stress at the core. When the core finally transforms, it tries to expand against the already-hardened surface. The result is a complex stress distribution that causes bending in asymmetric parts, ovality in rings and bores, and taper in long shafts. Scale and decarburization: the part surface oxidizes during furnace heating, forming an iron oxide scale layer (FeO, Fe₃O₄) typically 0.003–0.020 inch thick. Below the scale, a decarburized zone (0.010–0.060 inch) has lost carbon to surface oxidation. Both layers must be removed in post-hardening machining — they are harder than normal scale, unpredictable in thickness, and decarburized zones have inadequate hardness for the application if left on the surface. The post-hardening stock allowance must be large enough to clear all three effects: phase transformation growth, thermal gradient distortion, and scale removal (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).
What stock allowances are recommended for through-hardened alloy steel by part size?
Through-hardening (quench and temper) of alloy steel produces the most distortion of any common hardening process because the entire cross-section transforms, generating volumetric expansion and thermal gradient stresses throughout the section. Recommended stock allowances per side (the amount left on each surface, applied to both OD and ID surfaces) for 4140 and 4340 after oil quench and temper: for parts under 3-inch nominal diameter or thickness — 0.015–0.030 inches per side. For 3–6-inch diameter or thickness — 0.020–0.040 inches per side. For 6–12-inch diameter or thickness — 0.030–0.060 inches per side. For 12–24-inch diameter or thickness — 0.050–0.100 inches per side. For 24–48-inch diameter (crane wheels, large rings, kiln tires) — 0.060–0.120 inches per side. The allowances increase with section size because thermal gradient distortion scales with the temperature differential between the surface and core during quench — larger sections have steeper temperature gradients and more distortion. For water quench (more severe than oil quench, used for lower-alloy steels to achieve adequate hardness): increase the allowances by 25–50% relative to oil quench values for the same section size. For 1045 water-quenched: the lower alloy content combined with water quench produces distortion at the high end of the range. Face surfaces (faces perpendicular to the part axis): 0.010–0.020 inches per face is typically adequate regardless of part diameter — face distortion is primarily from convex bowing of the face during quench, which is smaller in magnitude than OD and bore distortion (ASM Handbook, Vol. 4A, ASM International, 2013).
What stock allowances are recommended for induction-hardened parts?
Induction hardening differs from through-hardening in that only the surface layer — typically 0.100–0.500 inches deep — is heated and quenched. The distortion from induction hardening is substantially less than from through-hardening because the thermal gradient is concentrated at the surface and the core remains at or near ambient temperature throughout the process, providing a rigid constraint on surface distortion. Recommended stock allowances per side for induction-hardened OD surfaces (tread surfaces, shaft journals): for parts under 6-inch diameter — 0.010–0.020 inches per side. For 6–18-inch diameter — 0.015–0.030 inches per side. For 18–48-inch diameter (crane wheel treads) — 0.020–0.060 inches per side. Bore surfaces after induction hardening: bore distortion from induction hardening of the OD is indirect — the compressive surface stress from tread hardening slightly reduces the bore diameter through hoop stress interaction. For crane wheels where the tread OD is induction hardened: leave 0.010–0.030 inches per side on the bore for post-hardening finish boring to correct this distortion. Scale formation from induction hardening is typically lighter than from furnace hardening (the heating time is shorter — 5–30 seconds vs. 30–90 minutes), but a decarburized surface still forms. The first post-hardening pass must clear the decarburized zone (typically 0.003–0.010 inch deep from induction heating) entirely, which sets the minimum post-hardening depth of cut at 0.010 inches regardless of the dimensional correction needed. UTEC Industrial induction-hardens crane wheel treads in-house and performs post-hardening finish turning where the specification requires it — maintaining the integrated machining and hardening workflow that eliminates inter-facility transport risk (ASM Handbook, Vol. 4A, ASM International, 2013).
How does carburizing and case hardening affect required stock allowances?
Case hardening by carburizing (heating in a carbon-rich atmosphere to diffuse carbon into the surface, followed by quench) produces different distortion patterns from through-hardening because the case depth is controlled (typically 0.030–0.150 inch) and the core remains softer. The distortion mechanism is similar to induction hardening in that the case expands during transformation while the core constrains it, but the distortion magnitude can be larger than induction hardening because carburizing involves long furnace cycles (2–12 hours at 1,600–1,750°F) with significant thermal gradients during heating, soak, and quench. Recommended stock allowances for carburized and case-hardened parts (e.g., 8620 crane wheel drive gears, case-hardened bores): for parts under 4-inch diameter — 0.015–0.030 inches per side. For 4–10-inch diameter — 0.025–0.050 inches per side. For over 10-inch diameter — 0.040–0.080 inches per side. The critical consideration for case-hardened parts: the post-hardening machining must not remove the hardened case entirely. The case depth (measured from the surface to the point where hardness drops below a specified value, typically 50 HRC) must remain intact after finish machining. If the drawing specifies a minimum case depth of 0.040 inch, and the post-hardening stock allowance per side is 0.030 inch, the pre-hardening machining must leave a surface with 0.070 inch of stock — enough for the finish machining to remove 0.030 inch and still leave 0.040 inch of case. This interaction between case depth specification and stock allowance planning requires the machinist and the heat treater to coordinate before the job begins (ASM Handbook, Vol. 4A, ASM International, 2013).
How should post-hardening finish machining be approached to avoid CBN insert breakage?
Post-hardening finish machining — turning or boring hardened steel at 45–62 HRC — with CBN inserts requires different process discipline from soft-state machining, and most CBN insert breakage is traceable to violations of the rules that govern hard turning setup and parameters. The first pass challenge: the first CBN pass on a post-hardening surface must cut through the scale layer (if present) and the decarburized zone to reach the uniform hardened substrate. Scale is abrasive and harder than the base hardened metal — it accelerates flank wear on the first pass. Minimum first-pass depth of cut: 0.010–0.015 inches, enough to clear scale and decarburization entirely. Starting at 0.005-inch depth on a scaled surface means the CBN is cutting scale on every revolution, dramatically reducing insert life. After clearing the scale on the first pass, subsequent passes of 0.005–0.015 inches are standard. The diameter runout of a heat-treated part is typically 0.005–0.020 inches — this means the first CBN pass depth varies by the runout amount around the circumference. A programmed depth of 0.015 inch on a 0.010-inch-runout part produces depths from 0.005 to 0.025 inch per revolution. CBN is brittle — heavy momentary cuts (0.025-inch depth at full CBN cutting speed) can chip the edge. The mitigation: on high-runout post-hardening parts, reduce the first pass cutting speed by 15–20% and use a tougher CBN grade (lower CBN content, ceramic binder) for the first pass, then switch to a harder grade for finishing passes once the surface is round and clean. Always verify the workholding runout before beginning CBN turning — runout above 0.005 inch in the chuck should be corrected by re-indicating the part before proceeding (ASM Handbook, Vol. 16, ASM International, 1989; Sandvik Coromant, Metalcutting Technical Guide).
What is the role of the stock allowance in managing distortion from asymmetric hardening?
Parts with asymmetric geometry — flanges machined on one end of a shaft, keyways or splines on part of the OD, or bores of different diameters at different cross-sections — distort asymmetrically during heat treatment because the thermal mass and hardenability vary around the part. A shaft with a large flange on one end will bow toward the flange during quench (the flange quenches slower than the shaft, transforming after the shaft and pulling it toward the flange side). A wheel with a hub on one side will bow away from the hub. These asymmetric distortions require a larger stock allowance on the surfaces that will be most affected — typically the bore (which can become oval or eccentric relative to the OD) and long OD surfaces (which can develop taper or bow). For a crane wheel with a 12-inch diameter bore and a 30-inch tread OD: the bore may distort 0.005–0.015 inch out of round after quenching, requiring the post-hardening bore finishing allowance to accommodate the maximum distortion at any point around the bore circumference. If the bore minimum inside diameter after quench is 11.930 inches (0.070-inch diametral shrinkage from the worst distortion point), the pre-hardening bore must be at least 11.930 inches to ensure that the post-hardening boring pass removes the distorted surface. The recommended pre-hardening bore for this case: 12.000 − 0.080 (allowance for 0.040 per side) = 11.920 inches — leaving 0.040 per side of stock, enough to clear the maximum distortion and the scale layer. Verifying this logic before machining (by consulting with the heat treater on expected distortion for the specific part geometry and hardening process) prevents the situation where the post-hardening finishing pass runs out of stock and leaves a distorted bore surface that cannot be corrected.
How does the machining sequence change when heat treatment falls between roughing and finishing?
When heat treatment occurs between roughing and finishing, the machining sequence must be planned to ensure the right amount of stock exists on every feature at every stage. A typical four-stage sequence for a through-hardened alloy steel part: Stage 1 — rough machine all features, leaving 0.100–0.200 inches per side beyond the post-hardening stock allowance (total stock: roughing stock + post-hardening stock). This removes the bulk of the material while the part is in the easy-to-machine soft state. Stage 2 — semi-finish machine all precision features to the post-hardening stock allowance: bores to 0.040–0.080 inches undersize, ODs to 0.040–0.080 inches oversize, faces to 0.010–0.020 inches excess. This produces a surface with controlled geometry (good roundness, low runout) that gives the hardening distortion a predictable starting point. Stage 3 — heat treat (quench and temper): the part distorts by up to the expected distortion amount, but all precision features have sufficient stock to absorb the distortion and provide material for the post-hardening finishing pass. Stage 4 — post-hardening finish machine: using CBN inserts for OD and ID features, correct all distortion and bring the part to final tolerance and surface finish. The machining sequence design is where UTEC's integrated machining and heat treating capability pays dividends — the machining team and the heat treating team are in the same facility and can communicate directly about expected distortion, stock allowance adequacy, and the parameters of the finishing passes, rather than relying on inter-company documentation and coordination between separate machine shop and heat treater vendors (ASM Handbook, Vol. 4A, ASM International, 2013; Machinery's Handbook, 31st ed., Industrial Press, 2020).
- Integrated Machining and Heat Treatment Workflows — the broader workflow context for in-house heat treatment
- Machining Hardened Steel: Tooling, Speeds, and Process Strategies — the CBN machining that works the post-hardening stock allowance
- Stress Relieving Machined Parts: When, Why, and How — related thermal treatment planning
- Annealing Before Machining: Why Material Condition Matters — pre-machining treatment that complements post-hardening planning
References
- ASM International. (2013). ASM Handbook, Volume 4A: Steel Heat Treating Fundamentals and Processes. ASM International.
- ASM International. (1989). ASM Handbook, Volume 16: Machining. ASM International.
- Machinery's Handbook, 31st ed. Industrial Press, 2020.
- Sandvik Coromant. Metalcutting Technical Guide. Sandvik Coromant.
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