Why the Decarburized Layer Is a Bolt's Hidden Killer: Causes, Detection and Control
Executive Summary
The decarburized layer is the easiest-to-miss defect on a bolt: normal appearance, qualified hardness, and on-spec dimensions, yet fatigue strength drops by more than 30%. This article explains the three levels and five causes of decarburization, quantifies the harm of decarburized layer depth, covers acceptance criteria and metallographic/hardness detection methods, and gives full-chain control measures and inspection-point recommendations from wire rod through spheroidizing annealing to heat treatment.
Table of Contents
- Why a Bolt That Looks Perfect Fails Early
- Causes and Three Levels of Decarburization
- Quantifying the Harm of the Decarburized Layer
- Standard Requirements and Acceptance Basis
- Detection Methods: Metallographic and Hardness
- Full-Chain Control: From Wire Rod to Finished Part
- Conclusion: Decarburization Is Hidden, but It Can Be Controlled
Why a Bolt That Looks Perfect Fails Early
There is a category of failure-analysis case that is especially puzzling: the bolt looks intact, has no corrosion or mechanical damage, and all dimensions and hardness are qualified, yet it fractures under alternating loads far below the design load. The fracture surface shows typical fatigue features, with the crack origin at the outermost surface of the thread root.
After metallographic sectioning, the answer emerges: there is a layer of about 0.15 mm of ferrite at the thread-root surface, with hardness only about 150 HV, while the core is normal 35 HRC tempered sorbite. This soft, weak layer is the decarburized layer.
Its harm lies in this: the bolt surface should originally be the hardest, strongest location (surface stress is highest under load), but after decarburization the surface becomes the weakest region across the whole cross-section. Under alternating load, the crack initiates right there.
Causes and Three Levels of Decarburization
Decarburization is the phenomenon in which surface carbon of steel is lost by oxidation or reaction with hydrogen in the atmosphere at high temperature. By degree it is divided into three levels:
| Type | Microstructure Feature | Effect on Properties |
|---|---|---|
| Complete decarburization | Surface entirely ferrite (no pearlite) | Most severe; hardness and fatigue strength drop sharply |
| Partial decarburization | Ferrite + partial pearlite (reduced pearlite) | Medium; surface hardness still below the matrix |
Common Causes
- Furnace atmosphere out of control: oxidizing atmospheres in the furnace (O2, CO2, H2O) continuously oxidize and carry away surface carbon.
- Excessive heating temperature or time: every 50 ℃ rise can multiply the decarburization rate.
- Decarburization inherited from raw material: wire rod surface often already has decarburization after rolling; if not fully removed before cold heading, it is retained on the finished part.
- Improper annealing and hot heading: spheroidizing annealing without a protective atmosphere, or high-temperature exposure during hot forming, are concentrated sources of decarburization.
Quantifying the Harm of the Decarburized Layer
The harm of the decarburized layer is not theoretical; it can be quantified:
| Decarburized Layer Depth | Surface Hardness | Fatigue Strength Loss (ref.) | Assessment |
|---|---|---|---|
| Below 0.02 mm | Near matrix | < 5% | Acceptable |
| 0.02～0.05 mm | Down 15%～25% | 5%～15% | Caution |
Besides fatigue strength, decarburization brings three knock-on effects:
- Reduced thread strength: thread load-bearing capacity depends on surface hardness; after decarburization, the thread flanks deform and wear easily.
- Insufficient surface hardness: affects wear resistance and anti-loosening performance.
- Reduced coating adhesion: the soft, porous surface layer affects plating quality.
Standard Requirements and Acceptance Basis
Decarburized layer depth is an important acceptance item for fasteners; the relevant standard system is as follows:
| Standard | Content | Key Requirement |
|---|---|---|
| GB/T 3098.1 | Mechanical properties of bolts, screws and studs | Specifies decarburization test methods and limits |
| GB/T 5779.1 | Surface defects of bolts, screws and studs — general requirements | Principles for surface defect judgment |
Typical Limits (per the GB/T 3098.1 system)
| Product Type | Complete Decarburization (G) | Partial Decarburization (incl. complete) |
|---|---|---|
| Thread nominal diameter ≤ M16 (class 8.8 and above) | Not allowed (0) | ≤ 0.015 mm |
| Thread nominal diameter > M16 | Not allowed | Per spec value (generally 0.02～0.03 mm) |
Detection Methods: Metallographic and Hardness
Metallographic Method
The most direct and commonly used method. Take a longitudinal section of the thread (through the thread axis), mount, grind, polish, and etch, then observe and measure under a metallographic microscope.
- Sampling: take at least the threaded portion, cut longitudinally (do not cut transversely, because the decarburized layer is distributed along the surface).
- Etching: use 2%～4% nital; ferrite appears white, tempered sorbite appears dark.
- Measurement: at 100～200×, measure the distance from the surface to where the microstructure returns to normal.
- Reading: measure separately at the thread crest, flank, and root, and take the worst (maximum) value as the acceptance basis.
Microhardness Method
Run a hardness gradient test from the surface toward the core (load 100～300 g) and plot a hardness-depth curve. The depth at which hardness returns to the matrix level is the decarburized layer depth; it applies to mild decarburization that is hard to judge metallographically. A practical approach is to use metallography as the arbitration basis and eddy current for in-line screening: an eddy current sorter can quickly remove heavily decarburized parts on the line, while metallographic sampling is used for batch release and process verification.
Full-Chain Control: From Wire Rod to Finished Part
Decarburization control must cover the whole process; improvement at any single stage alone is not enough to solve the problem completely.
| Stage | Control Measure | Key Indicator |
|---|---|---|
| Wire rod incoming | Inspect decarburized layer depth; reject if over limit | ≤ 0.5%～1.0% of diameter |
| Spheroidizing annealing | Use protective atmosphere or vacuum annealing | Furnace oxygen content, decarburized layer depth |
The Three Most Effective Shop-Floor Measures
- Control furnace carbon potential: keeping the furnace gas carbon potential slightly above or equal to the steel's carbon content effectively suppresses decarburization. This is the core control parameter for continuous mesh belt furnaces.
- Shorten high-temperature exposure time: pass quickly through the decarburization-sensitive temperature zone (above 700 ℃) during heating. When batch loading, control the load to avoid individual parts dwelling too long.
- Protective atmosphere or coating: where atmosphere furnaces cannot be used, apply anti-decarburization coating to the blank surface.
Conclusion: Decarburization Is Hidden, but It Can Be Controlled
The reason the decarburized layer is called a "hidden killer" is that it hardly changes the product's appearance, dimensions, or hardness—it only changes the surface microstructure. And bolt fatigue cracks initiate precisely from the surface.
The path to controlling decarburization is clear: control the raw material's decarburized layer, control the atmosphere in annealing and heat treatment, control high-temperature dwell time, and verify results with metallography and hardness. Do these four things well, and decarburization changes from "an occasional hidden defect" into "a controllable process indicator." For high-strength fasteners, this is a quality threshold as important as hardness inspection.
