Carburizing Case Depth Control: Prediction, Measurement, and Troubleshooting

Introduction: The Importance of Case Depth Control

Carburizing is a thermochemical heat treatment process that diffuses carbon into the surface of low-carbon steel at elevated temperature (typically 870–950°C) to create a high-carbon case that, after quenching, produces a hard, wear-resistant surface with a tough, ductile core. The depth of this case is the primary specification for most carburized components — too shallow and the component wears prematurely; too deep and it becomes brittle or the core properties are compromised.

Case depth control requires an understanding of carbon diffusion, furnace atmosphere control, and the relationship between process parameters (temperature, time, carbon potential) and the resulting carbon profile. Even modest deviations from the target case depth can have serious consequences: a gear with 0.5 mm case depth instead of the specified 0.8 mm may fail in service under load, while a shaft with 1.5 mm case instead of 1.0 mm may develop grinding cracks during finishing or fracture under impact loading due to the excessively deep brittle layer.

This guide covers the complete topic from diffusion theory through prediction, measurement, and troubleshooting for both gas carburizing and vacuum carburizing processes.

1. Defining Case Depth

Total Case Depth

Total case depth is the perpendicular distance from the surface to the point where the composition or hardness no longer differs measurably from the core. It is determined by chemical analysis (the point where carbon content returns to the base metal carbon level) or by metallographic examination (the point where the microstructure is entirely that of the core).

Effective Case Depth

Effective case depth is the more commonly specified parameter. It is defined as the perpendicular distance from the surface to the point where a specific criterion is met. The two most common criteria are:

• 50 HRC criterion: The depth at which hardness drops to 50 HRC (approximately 513 HV). This is the standard criterion in most automotive and general engineering specifications
• 0.40% carbon criterion: The depth at which the carbon content drops to 0.40%. Used when the specification is based on carbon profile rather than hardness

These two criteria do not always yield the same result because hardness depends on carbon content, alloying elements, quench severity, and tempering temperature. For critical applications, the specification should clearly state which criterion applies. A highly alloyed steel (e.g., SAE 9310 with 3.25% Ni, 1.2% Cr) develops higher hardness at a given carbon level than a lean alloy (e.g., SAE 8620), so the 50 HRC point falls at a lower carbon concentration and greater depth.

Relationship Between Total and Effective Case Depth

As a general approximation, total case depth is 1.4–1.6× the effective case depth (50 HRC criterion). This ratio varies with steel grade and process conditions, but it provides a useful estimate when only one measurement is available.

2. Carbon Diffusion Theory

Fick’s Laws of Diffusion (Simplified)

Carbon diffusion during carburizing is governed by Fick’s laws. The key practical relationships are:

• Fick’s First Law: The rate of carbon transfer across the surface is proportional to the concentration gradient. A higher difference between the atmosphere carbon potential and the surface carbon concentration drives faster carbon absorption
• Fick’s Second Law: The carbon concentration at any depth changes over time according to the diffusion coefficient and the concentration profile. The solution to this equation gives the S-shaped carbon profile that develops during carburizing

The practical consequence of Fick’s laws is that case depth is proportional to the square root of time at constant temperature. Doubling the case depth requires four times the process time. This square-root relationship is the fundamental planning tool for carburizing cycles. It also explains why deep cases (above 2.0 mm) require very long cycle times and why high-temperature carburizing (above 950°C) is attractive for deep-case applications despite the increased risk of grain growth.

Diffusion Coefficient

The diffusion coefficient of carbon in austenite (D) increases exponentially with temperature. Approximate values:

Increasing the carburizing temperature from 925°C to 955°C increases the diffusion rate by approximately 36%, reducing cycle time significantly. However, higher temperatures increase grain growth, distortion, and element wear, so there is an engineering trade-off. For diffusion coefficient calculations, see our Diffusion Calculator.

3. Predicting Case Depth

The Harris Formula

The Harris equation provides a practical method for estimating effective case depth based on time and temperature:

d = K √t

Where:

• d = effective case depth (mm)
• K = carburizing constant (dependent on temperature and definition of case depth)
• t = time at temperature (hours)

Typical values of K (for effective case depth to 0.40% C):

Example: To achieve 1.0 mm effective case depth at 925°C:

t = (d / K)² = (1.0 / 0.635)² = 2.48 hours at temperature

This is the time at the carburizing temperature with the carbon potential established — it does not include heat-up time, equalisation, or the diffuse portion of a boost-diffuse cycle.

Factors That Modify the Prediction

• Steel grade: Alloying elements affect both the diffusion coefficient and the carbon solubility. Chromium, manganese, and molybdenum shift the equilibrium and can increase or decrease the effective case depth for a given carbon profile
• Surface carbon potential: The Harris formula assumes a constant surface carbon concentration. Varying the carbon potential during the cycle (boost-diffuse) changes the profile and requires more sophisticated calculation
• Prior microstructure: A normalised or annealed starting structure gives more uniform diffusion than a cold-worked or banded structure

For detailed case depth predictions accounting for these factors, use our Case Depth Calculator.

4. Boost-Diffuse Cycles

A straight carburize at a high carbon potential (e.g., 1.10% C) produces the deepest case in the shortest time, but the surface carbon concentration may be too high — causing retained austenite, massive carbide networks, or excessive hardness gradients. The boost-diffuse cycle addresses this:

1. Boost phase: Carburize at a high carbon potential (0.9–1.2% C depending on steel grade) to drive carbon rapidly into the surface. This phase accounts for 60–80% of the total cycle time
2. Diffuse phase: Reduce the carbon potential to the target surface carbon level (typically 0.75–0.85% C) and hold. During this phase, no additional carbon enters from the atmosphere; instead, the steep carbon gradient causes carbon to diffuse inward, evening out the profile and reducing the surface concentration

The diffuse time is typically 20–40% of the boost time. The exact ratio depends on the target surface carbon, the boost carbon potential, and the desired case depth uniformity. Some modern furnace controllers calculate the boost-diffuse split automatically based on the target carbon profile using built-in diffusion models. For manual planning, a good starting point is a 3:1 boost-to-diffuse time ratio with the boost at 1.05–1.10% C and the diffuse at 0.80% C for a typical 0.80% C surface target.

For carbon potential control during boost-diffuse cycles, see our Carbon Potential Reference.

5. Measuring Case Depth

Microhardness Traverse

The definitive method for measuring effective case depth. A series of Vickers or Knoop hardness indentations are made on a polished cross-section of the carburized part, starting at the surface and progressing toward the core at defined intervals:

• Load: HV 0.3 or HV 0.5 (Vickers) or HK 0.5 (Knoop) per ASTM E384
• Spacing: Indentations at 0.05–0.1 mm intervals for cases up to 1.5 mm; 0.1–0.2 mm for deeper cases
• Interpretation: Plot hardness vs. depth and determine the depth at which hardness drops to 50 HRC (or specified criterion)
• Reporting: Effective case depth, surface hardness, core hardness, and the complete hardness traverse profile

Chemical Analysis (Carbon Profile)

Optical Emission Spectrometry (OES) with successive surface grinding or Electron Probe Microanalysis (EPMA) on a cross-section can determine the carbon concentration as a function of depth. This method directly reveals the carbon profile and is the only way to confirm that the surface carbon concentration matches the target. It is more expensive than microhardness and is typically reserved for process qualification or failure analysis.

File Test

A hardened file is drawn across the surface at various locations. The file will skate on hardened surfaces (>60 HRC) and bite into soft surfaces. While useful as a quick go/no-go check, the file test provides no quantitative measurement and should not be used as the sole acceptance criterion for critical components.

Non-Destructive Methods

Eddy current and magnetic Barkhausen noise (MBN) methods can estimate case depth non-destructively by detecting the change in electromagnetic properties at the case-core transition. These methods require calibration against destructive measurements and are used for 100% production screening after initial process qualification. For eddy current testing, calibration standards must be manufactured from the same steel grade, heat-treated to the same specification, and verified by destructive testing. A minimum of 5 calibration standards spanning the specification range is typical.

6. Factors Affecting Case Depth

7. Troubleshooting Case Depth Problems

Shallow Case

• Insufficient time at temperature: Verify that the furnace recorder shows the load reached the setpoint for the full cycle time. Thermocouple placement in the load (not just the atmosphere) is essential
• Low carbon potential: Check oxygen probe calibration, verify dew point with a portable analyser, check enrichment gas flow rate and supply pressure
• Air leaks: Even small air leaks dilute the atmosphere and reduce carbon potential. Check door seals, fan shaft seals, thermocouple entry points, and quench elevator seals
• Wrong steel grade: Confirm material certificates. A steel with higher carbon content (e.g., 0.25% C instead of 0.20% C) will produce a shallower effective case depth for the same process because the base carbon is closer to the criterion

Deep (Excessive) Case

• Over-time: Verify timer calibration and confirm that the cycle end is triggered by time at temperature, not elapsed time from start
• Temperature over-run: Check calibration of the process thermocouple and the controller. A thermocouple reading 10°C low will cause the furnace to operate 10°C high, increasing case depth
• Carbon potential too high: Verify oxygen probe reading against shim stock analysis or portable dew point measurement

Uneven Case Depth

• Temperature non-uniformity: Perform a TUS to verify that the furnace meets the required uniformity class. Non-uniform temperature directly produces non-uniform case depth
• Poor gas circulation: Atmosphere must be well-mixed to maintain uniform carbon potential throughout the work zone. Check fan operation, baffle positioning, and load spacing
• Load density too high: Parts packed too tightly restrict gas flow between them. Ensure minimum spacing between parts (typically ≥ 10 mm)
• Part orientation: Surfaces facing the gas flow may carburise faster than shielded surfaces. Fixturing should promote uniform exposure

8. Case Depth Specifications

Automotive

Automotive carburizing specifications (e.g., CQI-9, GM, Ford, Stellantis requirements) typically specify:

• Effective case depth (50 HRC criterion) with tolerance (e.g., 0.8 ± 0.15 mm)
• Surface hardness range (e.g., 58–63 HRC after tempering)
• Core hardness range (e.g., 30–45 HRC)
• Maximum retained austenite at surface (≤ 25% typical)
• No intergranular oxidation (IGO) below a specified depth (typically ≤ 0.013 mm in high-performance applications)

Aerospace

Aerospace carburizing specifications (AMS 2759/7, Boeing BAC 5617, Rolls-Royce MSRR 9712) are more stringent:

• Effective case depth with tight tolerance (e.g., 0.90 ± 0.10 mm)
• Surface carbon concentration specified (e.g., 0.75–0.90% C)
• Microhardness traverse required on every lot (or per part for flight-critical components)
• Metallographic examination for carbide networks, retained austenite, IGO, and microstructure quality
• Furnace must be qualified per AMS 2750 and Nadcap-accredited

9. Process Documentation and Control

Consistent case depth requires documented and controlled processes. Essential documentation includes:

• Process recipe: Temperature, time, carbon potential (boost and diffuse), quench medium and conditions, tempering temperature and time. See our Heat Treatment Recipes for recipe templates
• Furnace records: Continuous temperature and carbon potential recording throughout the cycle
• Material traceability: Steel grade, heat number, and supplier certification for every load
• Quality records: Case depth measurements, hardness results, metallographic reports
• Corrective action: Documented investigation and corrective action for any out-of-specification result

Statistical process control (SPC) of case depth measurements over time provides early warning of process drift — a gradually increasing or decreasing trend signals a systematic change (thermocouple drift, element degradation, atmosphere system wear) that can be corrected before parts are scrapped. X-bar and R charts plotting effective case depth from each production lot, with control limits derived from initial process capability data, are the standard SPC tools. A process capability index (Cpk) of 1.33 or greater is typically required for automotive applications, while aerospace specifications may require Cpk ≥ 1.67.