Carburizing Surface Hardness: How to Specify Case Depth and Core Hardness

When you are sourcing carburizing services for your parts, the conversation around hardness can get confusing fast. Effective case depth, total case depth, surface hardness, core hardness, these terms get used interchangeably in emails and on quote forms, but they mean very different things to a heat treater. Getting them right on your request for a quote is not just good practice. It directly determines whether the parts you receive perform the way you need them to.

What Carburizing Actually Does to a Part

Carburizing is a case hardening process that introduces additional carbon into the surface layer of a low carbon steel part. The base material, something like 8620, 1018, or 1045, starts with relatively low carbon content, which means it would not achieve meaningful hardness through conventional through hardening alone. The carburizing process exposes the part to a carbon rich atmosphere at elevated temperature, allowing carbon to diffuse into the surface over time.

Once the part is quenched, that carbon enriched surface transforms into hard martensite, while the lower carbon core remains tough and ductile. The result is a part that resists wear and contact fatigue on the outside while retaining the ability to absorb shock and bending loads on the inside. That combination is exactly what gears, shafts, pins, and blades need to survive in demanding applications.

The depth and hardness of that case is not automatic. It is a function of time, temperature, atmosphere carbon potential, and the base material you start with. That is why your specification matters so much as it tells the heat treater how long to run the cycle and what result to verify before shipping your parts.

Surface Hardness: What You Are Targeting at the Very Top

Surface hardness refers to the hardness measured at or very near the outermost layer of the carburized case. For most carburized steels, a properly executed process will achieve surface hardness in the range of 58 to 63 HRC, with some materials and carbon levels reaching 65 HRC under the right conditions.

A few things worth knowing when you specify surface hardness:

• The steel grade matters. An 8620 part and a 1018 part will both carburize well, but their achievable surface hardness values differ slightly due to differences in alloy content and hardenability. Make sure your target hardness is achievable with your chosen material before you build it into a drawing requirement.
• Measurement location matters. Surface hardness is typically tested on a representative coupon or on a finished test surface of the actual part. Specify whether you need the measurement taken before or after any post processing such as grinding or shot blasting.
• Tempering affects the final number. Most carburized parts are tempered after quenching to relieve brittleness in the case. A higher tempering temperature will reduce surface hardness. If you need 62 HRC minimum, communicate that so the temper cycle is set accordingly.

Effective Case Depth vs. Total Case Depth

This is where most buyers run into trouble, and it is worth spending some time here. Effective case depth and total case depth are both valid ways to describe how deep the hardened layer goes, but they measure different things and should not be used interchangeably on a print or RFQ.

Effective case depth (ECD) is defined as the depth at which the hardness falls to a specified cutoff value — most commonly 50 HRC, though some specifications use 52 HRC or a different threshold. You might see this written as ECD to 50 HRC or simply effective case depth with a reference to a governing specification. This is the most practical measurement for functional applications because it tells you how deep the part is meaningfully hard in service.

Total case depth (TCD) is the full depth of the carbon enriched zone, measured from the surface to the point where carbon content returns to the base metal level. Total case depth is always deeper than effective case depth on the same part. It is more commonly used in metallurgical analysis and cross section evaluation than in day to day functional specifications.

When you write a carburizing requirement without specifying which measurement you mean, a heat treater has to make an assumption. If they assume total case depth and you meant effective case depth, your parts could come back appearing to meet the number while being shallower in functional hardness than you need. Clarity here prevents that outcome entirely.

Core Hardness: The Other Half of the Specification

Core hardness is the hardness of the base material at the center of the part, well below the carburized case. It is governed almost entirely by the chemistry of the steel and the quench rate, not by the carburizing cycle itself.

For a low carbon carburizing grade like 8620, a typical core hardness after quench and temper falls in the range of 25 to 40 HRC depending on section size and quench media. For 1018, core hardness will be lower due to the lower alloy content and reduced hardenability.

Why does core hardness matter? Consider a gear tooth under load. The hard case resists surface wear and pitting, but the core beneath it needs enough strength to prevent the tooth from yielding or fracturing at its root. If the core is too soft, the part can deform under load even if the surface looks fine. If the core is harder than necessary, the part may become more brittle overall and more prone to catastrophic fracture rather than gradual wear.

When you specify core hardness, include both a minimum and a maximum. A range like 28 to 38 HRC gives your heat treater a target band that is achievable and meaningful. A single minimum without a ceiling is often not enough guidance, especially for toughness critical applications.

How to Write a Carburizing Specification That Works

A complete carburizing callout on a drawing or RFQ should include all of the following elements. Missing even one can lead to back and forth, incorrect processing, or parts that pass inspection but fail in service.

1. Base material and condition. Specify the steel grade and its incoming condition (annealed, normalized, pre hardened). The heat treater needs this to establish the correct cycle parameters and predict achievable results.
2. Surface hardness range. Provide a minimum and maximum, such as 60 to 64 HRC. If the application requires a specific tempering temperature to achieve retained toughness, note that as well.
3. Case depth type and value. State whether you are specifying effective case depth or total case depth, and give a range rather than a single number. For example: effective case depth 0.020 to 0.035 inch to 50 HRC cutoff. This is the most commonly misunderstood part of a carburizing callout, so precision here pays off.
4. Core hardness range. Give the acceptable window for core hardness with both a floor and a ceiling, such as 25 to 40 HRC.
5. Areas to mask or leave soft. If any threads, bores, or features need to remain machinable or are to be carburized but not hardened, call those out explicitly. Copper plating and stop off paints are common methods, and identifying these areas upfront avoids rework.
6. Governing specification, if applicable. If your drawing references AMS 2759/7 or a customer supplied specification, include that document. It sets testing methods, acceptance criteria, and documentation requirements that take precedence over general practice.

Common Mistakes That Lead to Rejected Parts or Failed Components

Even experienced engineers occasionally send carburizing requirements that create problems down the line. Here are the situations that come up most often:

• Specifying case depth without defining ECD or TCD. As discussed above, this creates ambiguity and invites misinterpretation. Always state which measurement applies.
• Setting unrealistic surface hardness targets for the chosen material. Asking for 65 HRC surface hardness on 1018 steel is possible under certain conditions, but it requires specific process controls and the right starting material condition. Confirm achievability before putting the number on the drawing.
• Omitting core hardness entirely. If the drawing only specifies case depth and surface hardness, the heat treater has no target for the core. They may achieve acceptable case results while the core ends up outside of what the application needs.
• Not accounting for stock removal after heat treat. If your parts will be ground or finished after carburizing, the finished case depth will be shallower than the as processed depth. Build in allowance if the drawing tolerance applies to the finished part.
• Applying the same specification to very different section sizes. A 0.5 inch diameter pin and a 3 inch diameter shaft made from the same steel grade will have very different core hardness outcomes from the same quench cycle. Adjust your core hardness expectations based on section size and the hardenability of your alloy.

Start Your Next Carburizing Project on the Right Foot

Writing a solid carburizing specification is one of the highest leverage things you can do before you submit a quote request. It reduces back and forth, prevents misprocessed parts, and gives you a clear basis for acceptance when your order arrives. If you are unsure how to specify case depth or core hardness for your application, the team at Southwest Metal Treating is ready to help you work through it. Reach out with your material, geometry, and application details, and we will help you build a specification that gets your parts processed correctly the first time. Talk to an engineer today and take the guesswork out of your next carburizing project.