Through Hardening Steel: Practical Guidance for Design Engineers

Heat treating is one of the most powerful tools available to mechanical designers—and also one of the easiest to misuse if expectations aren't aligned with material reality. Through hardening, in particular, is often assumed to be a way to “make steel very hard” without fully considering how alloy choice, section size, and carbon content govern the outcome.

This page focuses specifically on through hardening steels: what it means, what controls achievable hardness, and how commonly used alloys like 4140, 4340, and 52100 behave in real machine components. Other processes such as carburizing and nitriding are mentioned briefly for context, but they are intentionally not the focus here.

What Through Hardening Actually Means

Through hardening is a heat treatment process intended to harden the entire cross-section of a steel part, not just the surface.

In simplified terms, the process involves:

1. Heating the steel above its critical temperature to form austenite
2. Quenching (oil, polymer, or sometimes water) to transform the structure to martensite
3. Tempering to reduce brittleness and adjust final hardness

When done correctly—and on the right material—the hardness is reasonably uniform from the surface to the core.

This is fundamentally different from surface-hardening processes, where only a thin outer layer is hardened while the core remains soft and tough.

Carbon Content Sets the Ceiling

A key point that often isn't emphasized early in an engineer's career:

Peak achievable hardness in steel is primarily governed by carbon content.

As a practical rule of thumb:

• Below ~0.40% carbon, true through hardening is not realistic
• Above ~0.40% carbon, martensite formation becomes sufficient for meaningful hardness
• Increasing carbon increases *maximum* hardness, not toughness

That's why low-carbon steels (1018, 1020, A36) are not through-hardened in practice, no matter how aggressive the quench.

Through Hardenability vs. Hardness

It's important to separate two related but different concepts:

• Hardness: How hard the steel can get (mostly carbon-driven)
• Hardenability: How deep that hardness can penetrate (alloy-driven)

Alloying elements like chromium, molybdenum, and nickel slow down transformation during cooling, allowing thicker sections to harden all the way through. This is why alloy steels outperform plain carbon steels in real components.

Common Through-Hardening Steels in Machinery

4140 (≈ 0.40% Carbon)

4140 is often the entry point into through-hardened alloy steels.

Practical characteristics:

• Minimum carbon content for meaningful through hardening
• Typical hardness range after heat treat and temper: ~28-40 HRC
• Good balance of strength, toughness, machinability, and cost
• Commonly used for shafts, pins, gears, and structural machine components

4140 can be through hardened in moderate section sizes, but very thick parts may still show a softer core. In many designs, this is acceptable—or even desirable.

4340 (≈ 0.40% Carbon, Higher Alloy Content)

4340 has similar carbon content to 4140, but significantly higher nickel content.

Why that matters:

• Much higher hardenability
• Better core hardness in thick sections
• Excellent toughness at high strength levels

Typical applications:

• Highly loaded shafts
• Aerospace and heavy-duty mechanical components
• Parts where toughness matters as much as strength

If a part is too thick to harden uniformly in 4140, 4340 is often the next step—not because it gets harder, but because it hardens more deeply.

52100 (≈ 1.00% Carbon)

52100 is a different class of steel entirely.

Key characteristics:

• Very high carbon content
• Capable of very high hardness (60+ HRC)
• Excellent wear resistance
• Lower toughness compared to 4140/4340
• Limited section size for full through hardening

This steel is most commonly associated with bearing components, rollers, and wear-critical parts. It is chosen when hardness and wear resistance dominate the design requirements.

Section Size Matters More Than Many Expect

Even with good alloy selection, through hardening is not unlimited.

• Thin parts harden more uniformly
• Thick sections cool more slowly
• Large cross-sections may have a hardness gradient, even in alloy steels

This is why heat treat specifications should always be considered alongside geometry, not in isolation. A 1-inch shaft and a 6-inch block made from the same steel will not behave the same way in heat treatment.

Tempering: Where Design Intent Shows Up

As-quenched martensite is extremely hard—and extremely brittle. Tempering is what makes through-hardened steel usable.

Tempering:

• Reduces brittleness
• Trades some hardness for toughness
• Allows engineers to tune properties to the application

Two parts made from the same steel, hardened the same way, can behave very differently depending on tempering temperature. That's why hardness ranges (not single numbers) are typically specified.

What Through Hardening Is Good At

• Uniform strength through the section
• Good fatigue performance
• Predictable mechanical properties
• Structural components under cyclic or impact loading

Through hardening is often the right choice for shafts, pins, gears, and load-bearing machine elements.

Limitations and Trade-Offs

Distortion is real

Quenching introduces stress. Long, thin, or asymmetric parts can warp and often require post-heat-treat machining.

Machinability drops significantly

Once hardened above ~35 HRC, machining becomes slower and more expensive. Many parts are rough-machined, heat treated, then finish-machined. However, for parts in the 28-32 HRC range can often be made form "pre-hard" 4140 or 4340 and machined in one go. Staying in this range keeps costs down rather than the rough -> harden -> finish process needed for higher hardness targets.

Hardness is not infinite

If a design truly requires very high surface hardness and a tough core, through hardening alone may not be the right solution.

Brief Note on Other Heat Treating Processes

Other processes are often mentioned alongside through hardening:

• Carburizing: Adds carbon to the surface to create a hard case with a tough core
• Nitriding: Diffuses nitrogen into the surface for wear resistance with minimal distortion

Both are powerful tools, but they solve different problems and deserve separate discussion.

When to Choose Through Hardening

Through hardening is typically a good choice when you need:

• Strength and fatigue resistance through the entire part
• Predictable mechanical behavior
• Good balance between hardness and toughness
• A material solution rather than a surface-only fix

When wear resistance is localized, or when dimensional stability is critical, surface-hardening processes may be more appropriate.