Stress Relief for Cast Iron: When & How to Specify (+ Normalizing)

Guided by YB Metal (Suzhou, China) — stress relief for cast iron: gray/grey & ductile iron castings with simulation-led gating, in-house machining, controlled heat treatment, and traceable QA.

Open Gray Iron Specs
Open Ductile Iron Specs
Open ADI Ductile Specs

Excerpt: This guide shows when to use stress relief for cast iron and when to step up to normalizing. You’ll get process windows (temperatures & hold times), section-size limits, hardness/size change expectations, warpage controls, and copy-paste RFQ notes.

Table of contents

  • Stress relief vs normalizing — what’s the difference
  • Selection matrix (when to use which)
  • Process windows by material (gray vs ductile)
  • Dimensional change & hardness — what to expect
  • Warpage control: fixtures, ramps, and size checks
  • Drawing & RFQ notes (paste-ready)
  • Inspection & documentation plan
  • FAQs

Stress relief vs normalizing — what’s the difference

  • Stress relief: Low-temperature soak that reduces residual casting/machining stresses with minimal microstructure change. Typical result: distortion risk drops; hardness and tensile stay near as-cast.
  • Normalizing: High-temperature austenitize + air cool to a more uniform pearlitic matrix. Typical result: higher hardness/strength, better dimensional stability for machining, and more consistent properties across sections.

Selection matrix — when to use which

Table A — Heat treatment choice by scenario

ScenarioChoose Stress ReliefChoose Normalizing
Parts warp after rough machining✅ First
Leak-tight housings drifting flatness after assembly torque✅
Need higher strength/wear than as-cast✅
Mixed microstructure across sections (soft/hard patches)✅
Thin ribs next to thick bosses (residual stress risk)✅✅ (if property uniformity also needed)
PPAP/FAI dimensional stability before finish machining✅✅ (per property target)

Process windows by material (practical ranges)

Indicative shop ranges; confirm with your foundry/heat treater for your section size and chemistry.

Table B — Stress relief windows

MaterialTemperature °C (°F)Hold time*CoolNotes
Gray/grey iron500–600°C (932–1112°F)1–2 h + ~1 h per 25 mm (1 in) sectionStill airMinimizes microstructure change; good for housings/frames
Ductile iron (A536)525–650°C (977–1202°F)1–2 h + ~1 h per 25 mm (1 in)Still airStay ≤650°C to avoid nodularity/matrix shifts

*Rule of thumb; very massive parts may require longer soaks.

Table C — Normalizing windows

MaterialAustenitize °C (°F)SoakCoolResult
Gray/grey iron860–900°C (1580–1650°F)~1 h per 25 mm (1 in) sec.Air to room tempMore pearlite → higher hardness/strength
Ductile iron (A536)880–920°C (1616–1688°F)~1 h per 25 mm (1 in)AirUniform pearlite; properties toward 80-55-06 style

Cautions:

  • Overheating ductile iron (>940–950°C) or over-soaking can coarsen graphite and reduce nodularity.
  • Always specify max ramp rates and support/fixturing for thin walls to keep geometry.
About YBmetal CNC workshop

Dimensional change & hardness — what to expect

Table D — Notes that save cost and pain

TopicCopy-paste note
Process & CTCasting route: Green sand (or Resin/Shell). Target ISO 8062-3 CT9 (or CT8/CT6) for as-cast geometry.
Datum policyFunctional features to be related to machined datums where possible. As-cast datums used only when no machined datum exists.
FlatnessAs-cast flatness per Table B. Machined flatness 0.05–0.20 mm by size.
PositionMachined holes use MMC where clearance allows. Cored holes are non-locating unless noted.
Profile*As-cast external surfaces: profile 1.0–2.0 mm (process-dependent), relative to A
Stock & finishMachining allowance per mass range; machined faces Ra 3.2–6.3 μm (125–250 μin).
Inspection packProvide CMM report for criticals, hardness/chemistry certs, and leak/NDT if applicable.
About YBmetal Products

Warpage control — fixtures, ramps, size checks

  • Fixturing: Support thin plates/rims on flat tooling; don’t clamp hard—allow free shrinkage.
  • Ramp rates: Heat/cool ≤ 100°C/h (≤ 180°F/h) for sensitive parts unless proven otherwise.
  • Soak parity: Use thermal couples on thick spots for massive housings.
  • Datums & checks: CMM datums first, then flatness/position; map before/after to learn part behavior.
  • Leak-tight parts: Perform leak test again after HT (0.8–1.5 bar, 30–60 s) to confirm seal faces didn’t move.
About YBmetal Quality Control

Drawing & RFQ notes（paste-ready）

Table E — Copy-paste specification snippets

TopicNote text
Stress reliefStress relieve at 525–650°C (977–1202°F) for 1–2 h + 1 h/inch, still air cool. Control ramp ≤ 100°C/h. Re-inspect datums & flatness.
NormalizingNormalize at 880–920°C (1616–1688°F), soak 1 h/inch, still air cool. Re-cut machining datums after HT.
SequenceRough machine → HT → finish machine. Keep machining stock per mass range.
InspectionProvide CMM of datums/flatness before & after HT; hardness map on 3 zones; leak test for housings.
FixturingHeat-treat on support fixtures; no hard clamps on thin sections.
RecordsSubmit furnace charts & time-at-temp, part ID traceable to lot.
About YBmetal Casting Methods

Inspection & documentation plan

Table F — Pilot-run plan (illustrative)

ItemMethodSampling
Hardness (HBW)Brinell on defined pads5 pcs/lot
Flatness (machined)CMM/sweep indicator5 pcs/lot; Cpk ≥ 1.33 after HT
Position (critical holes)CMM to AB
Leak test (if applicable)Air 0.8–1.5 bar, 30–60 s100% through T1
Furnace recordChart + thermocouplesEach load
VisualAQLEach lot

Why YB Metal for stress relief & normalizing

  • One roof: green sand/resin/shell + machining; qualified heat-treat partners
  • Controlled ramps & soaks with serialized furnace charts
  • Before/after CMM + hardness maps; leak/NDT evidence for housings
About YBmetal Casting Methods

FAQs

Only slightly. Its goal is distortion control, not property change; expect small hardness increases at most.

When you need more uniform hardness/strength, or when a mixed microstructure causes inconsistent machining or field behavior.

No. Use HT when the risk/cost of distortion or property scatter justifies it—see the selection matrix.

If overheated or soaked too long near austenitizing temperatures. Stay in the 525–650°C band for stress relief and ~880–920°C for normalizing with proper control.

Rough guide is 1 hour per inch (25 mm) of effective section at temp, plus controlled ramps and air cool.

CTA — heat treat with proof, not promises

YBmetal delivers stress-relieved and normalized iron castings with documented process control so engineering and SQE can sign off fast.

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