Measuring Decarburization and Carburization Depth in Steel

What is decarburization in steel?#

Decarburization is the loss of carbon from the surface layer of a steel part during exposure to oxidizing atmospheres at elevated temperatures, such as during hot rolling, forging, or heat treatment. The carbon near the surface reacts with oxygen, hydrogen, or carbon dioxide, leaving a zone that is leaner in carbon than the bulk material. Because carbon content directly controls hardenability and strength in steels, a decarburized surface is softer and weaker than the core.

The depth of the affected zone is a critical quality metric. Even a few thousandths of an inch of decarburization can reduce fatigue life, lower surface hardness, and impair wear resistance. Specifications therefore set maximum allowable depths, and laboratories must measure them with precision.

What is carburization in steel?#

Carburization is the diffusion of carbon into the surface from a carbon-rich environment, either intentionally or unintentionally. While intentional carburizing creates wear-resistant case-hardened layers, unintended carburization during heat treatment can alter surface properties and must be measured against specification limits. The same measurement principles apply to both decarburization and carburization, but decarburization is far more commonly evaluated on finished parts.

What are the three types of decarburization?#

ASTM E1077 and common metallographic practice recognize three distinct forms of decarburization: complete decarburization (a ferrite rim), partial decarburization (a graded zone), and ferrite banding (grain boundary decarburization). Each type has a distinct microstructural appearance and requires a specific measurement approach to quantify the affected depth.

Complete Decarburization (Ferrite Rim)#

Complete decarburization appears as a continuous layer of ferrite grains at the surface, entirely free of pearlite, bainite, or martensite. In hypoeutectoid steels, this zone is pure ferrite with no visible carbon-rich phase. The boundary between the fully decarburized layer and the underlying partial zone or core is usually sharp. In some alloy steels, the layer may contain a small amount of carbide formers, but the matrix is essentially carbon-free ferrite.

Measurement of complete decarburization depth is straightforward metallographically: the distance from the surface to the innermost point where the structure changes to a two-phase or hardened structure.

Partial Decarburization#

Partial decarburization is a graded zone where the carbon content decreases progressively toward the surface but has not dropped to zero. Under the microscope, you see a mixture of ferrite and the carbon-rich phase (pearlite, bainite, or tempered martensite) with the proportion of ferrite increasing closer to the edge. In hardened and tempered steels, partial decarburization often appears as a light-etching zone with less dark-etching carbide.

The boundary of partial decarburization is defined by the point where the microstructure becomes indistinguishable from the unaffected core. This transition can be subtle, which is why microhardness is often preferred for quantification.

Ferrite Banding (Grain Boundary Decarburization)#

Ferrite banding, sometimes called grain boundary decarburization, occurs when carbon depletion follows prior austenite grain boundaries. It appears as a network of ferrite outlining the grain structure near the surface, often penetrating deeper than uniform complete decarburization. This type is common in medium-carbon steels that were heated in a decarburizing atmosphere and then cooled slowly enough for ferrite to nucleate on grain boundaries.

Measuring the depth of ferrite banding requires tracing the deepest penetration of the ferrite network. ASTM E1077 notes that this form is often reported separately from uniform decarburization because it can act as a stress riser and affect fatigue properties differently.

How should samples be prepared to avoid obscuring the decarburized zone?#

The most common error in decarburization measurement is sample preparation that alters or hides the true surface condition. A decarburized layer can be as thin as 0.001 in. (25 µm), so even slight edge damage can make the zone unreadable. Proper mounting, grinding, polishing, and etching are essential to preserve the surface for accurate measurement.

Mounting#

Use a mounting medium that adheres tightly to the specimen edge and does not shrink during curing. Epoxy hot-mounting compounds with fine filler particles give the best edge retention. Avoid phenolic powders that pull away from the surface on cooling. For small or irregular parts, consider plating the surface with a thin layer of nickel or copper before mounting to protect the edge mechanically and provide a clear optical boundary.

Grinding and Polishing#

Aggressive grinding rounds the specimen edge, removing the very material you need to measure. Use a semi-automatic or automatic polisher with a fixture that holds the sample perpendicular to the platen. Start with the finest grit that will plane the surface without excessive material removal; 320-grit SiC paper is often sufficient for initial planing if the cut surface is already flat. Progress through 400, 600, and 1200 grit, then polish with diamond suspensions down to 1 µm. Keep grinding times short and inspect the edge after each step. If edge rounding appears, re-mount the specimen or use a harder polishing cloth.

Etching#

Over-etching can dissolve the very surface you are trying to measure, especially in the ferrite layer. Use a 2% nital solution (2 mL nitric acid in 98 mL ethanol) for most carbon and low-alloy steels. Swab the specimen for only a few seconds, rinse immediately, and inspect. If the structure is faint, re-etch briefly rather than leaving the etchant on for a long period. For high-alloy steels, a picral etch or a tint etch may give better contrast without attacking the ferrite.

After etching, examine the edge under a microscope at 100x to 500x magnification. The true surface should be sharp and continuous. If the edge appears rounded, pitted, or pulled away from the mount, the measurement will not be reliable, and the specimen must be re-prepared.

What measurement methods does ASTM E1077 provide for decarburization depth?#

ASTM E1077 provides two distinct methods for measuring decarburization depth: the microhardness traverse method and the metallographic method. The microhardness method measures hardness as a function of distance from the surface, while the metallographic method relies on direct measurement of the visibly altered layer under a microscope. The choice depends on material condition, specification requirements, and the type of decarburization.

Microhardness Traverse Method#

The microhardness method measures hardness as a function of distance from the surface. A series of indentations is made along a line perpendicular to the surface, typically using a Vickers or Knoop indenter with a light load (100 gf to 500 gf) to keep the indent small enough to resolve gradients. The test is covered in detail in ASTM E384, and the procedure for case depth measurement is similar to that described in our article on Microhardness Testing for Case Depth and Coatings.

The depth of decarburization is defined as the distance from the surface to the point where the hardness reaches a specified limit. This limit may be a fixed hardness value (e.g., 50 HRC equivalent), a percentage of the core hardness (e.g., 85% of core), or a value stated in the material specification. The traverse must extend far enough into the material to establish a stable core hardness plateau.

Key points for a valid traverse:

• Indent spacing should be at least 2.5 times the indent diagonal length to avoid work-hardening interference.
• The first indent should be as close to the edge as practical, typically 0.001-0.002 in. (25-50 µm) from the surface.
• A minimum of five indentations in the affected zone is recommended to define the gradient reliably.
• Plot hardness versus distance and interpolate the depth at the specified limit.

Metallographic Method#

The metallographic method relies on direct measurement of the visibly altered layer under a calibrated microscope. After proper preparation and etching, the operator identifies the boundary between the decarburized zone and the unaffected core and measures the perpendicular distance from the surface to that boundary. An eyepiece reticle or image analysis software can be used.

For complete decarburization, the boundary is usually clear. For partial decarburization, the specification must define the criterion for the boundary, such as the point where the microstructure matches the core or where the ferrite fraction falls below a certain threshold. When the criterion is subjective, the microhardness method is preferred.

The metallographic method is faster and works well for quality control of known materials, but it requires a skilled metallographer and a well-defined acceptance standard.

How should decarburization measurement results be reported and what acceptance criteria apply?#

A decarburization measurement report must state the measurement method, the depth measured, the specification limit, and a conformance statement. Typical entries include the type of decarburization, specimen location, and microhardness load if used. Acceptance criteria are set by material specifications such as AMS 2759/1 and ASTM A574, often limiting total decarburization to 0.003 in. or less.

When reporting, always include:

• Material identification and heat treatment condition.
• Specimen location (transverse section, longitudinal, specific feature).
• Measurement method and load (if microhardness).
• The type of decarburization measured (complete, partial, ferrite banding).
• The measured depth and the specification limit.
• A statement of conformance or non-conformance.

Many AMS and ASTM specifications set maximum decarburization limits. For example, AMS 2759/1 for heat treatment of low-alloy steel parts often limits total decarburization to 0.003 in. or less. ASTM A574 for alloy steel socket-head cap screws typically allows no complete decarburization and partial decarburization not exceeding 0.005 in. on the thread flanks. Procurement teams should know the applicable specification and ensure the test report addresses the exact requirement.

Understanding how decarburization fits into the broader mechanical behavior of steel is also important. A part with excessive surface decarburization will show lower apparent hardness and may fail fatigue tests even if core properties are acceptable. Our Mechanical Testing of Metals guide explains how surface condition affects tensile and fatigue results.

Key takeaways#

• Decarburization is carbon loss at the surface; carburization is carbon gain. Both alter surface properties and must be measured against specification limits.
• Three types exist: complete (pure ferrite layer), partial (gradient zone), and ferrite banding (grain boundary networks). Each requires specific measurement approaches.
• ASTM E1077 provides two primary methods: microhardness traverse (objective, preferred for partial decarb) and metallographic measurement (faster, good for complete decarb).
• Sample preparation is the single largest source of error. Edge rounding, over-etching, and mounting shrinkage can hide the true surface; use epoxy mounts, gentle polishing, and brief nital etching.
• Microhardness traverses must use proper indent spacing, start close to the edge, and establish a core hardness baseline. The depth is reported at a specified hardness limit or percentage of core.
• Metallographic measurement requires a clear definition of the boundary, which is straightforward for complete decarb but subjective for partial.
• Report results with method, load, depth, specification limit, and conformance statement. Common specifications like AMS 2759/1 and ASTM A574 set maximum depths often in the 0.001-0.005 in. range.
• For further detail on grain structure effects, see our article on Grain Size Measurement: ASTM E112 Comparison and Planimetric Methods.