Fastener Guide / Engineering & Failure Analysis

Heat Treatment and Hardness Problems

Heat treatment sets strength. Wrong hardness, too soft or too hard, and surface decarburization are common, testable causes of failure.

Last reviewed 2026-09-10 · Governing standard: ASTM F606/F606M-26a; SAE J429 (2014); ISO 898-1:2013 · reference FG-9625AC · Report an error on this sheet

Common problems

ProblemResultCheck
Too softStretching, stripping, low strengthRockwell hardness vs spec range
Too hardBrittle fracture, HE susceptibilityRockwell hardness vs max
DecarburizationSoft surface, fatigue cracks at threadsMicrohardness traverse at thread
Quench cracksCracks from heat treatVisual, magnetic particle

These tables carry the same figures as the printed sheet below.

The printed sheet

Heat Treatment and Hardness Problems

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Text version of this sheet

Heat treatment and hardness problems

What it is

FAILURE MODE

HRC

Heat treatment sets strength. Wrong hardness, too soft or too hard, and surface decarburization are

common, testable causes of failure.

Hardness tells you if heat treat was right

Common problems

Problem

Result

Check

Too soft

Stretching, stripping, low strength

Rockwell hardness vs spec range

Too hard

Brittle fracture, HE susceptibility

Rockwell hardness vs max

Decarburization

Soft surface, fatigue cracks at threads

Microhardness traverse at thread

Quench cracks

Cracks from heat treat

Visual, magnetic particle

Key points

Hardness ranges are in the product standard: e.g. SAE J429 Grade 8 = C33 - C39.

Governing standards: ASTM F606/F606M-26a; SAE J429 (2014); ISO 898-1:2013. Editions shown are the latest confirmed as of September 2026. The drawing and the governing

standard always take precedence over this sheet.

torquing any fastener. Verify the current governing standard, project documents, OEM requirements, size, thread, coating and lot certification. To the fullest extent permitted by law, The Fastener App disclaims liability

arising from reliance on this reference.

Fastener Guide reference FG-9625AC · thefastenerapp.com/fastener-guide/failure-analysis/heat-treatment-and-hardness-problems · © The Fastener App. Reference only — verify the current governing standard, drawing and manufacturer data before specifying or installing.

Related sheets

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