A Heat Treatment Lesson from a Custom Caster Ring: Why 45# / S45C / AISI 1045 Hardness Can Vary

At CIXI HOTO, customized components often teach us more than standard products.

Recently, we manufactured a customized inner ring for a caster application. The machining itself was straightforward, but the heat treatment result gave us an important lesson about material selection, hardenability, and how carefully technical requirements should be reviewed before production.

The Project

Product: Customized caster inner ring

Inner Diameter: 32 mm

Outer Diameter: 55.6 mm

Material: 45# Steel / S45C / AISI 1045

Process: CNC Turning + Heat Treatment

Required Hardness: 38–42 HRC

After heat treatment, however, hardness measurements taken at different positions on the same end face showed a range of approximately:

35–43 HRC

The average hardness was close to the target, but the variation was too large to consistently meet the customer's specified 38–42 HRC range.

This raised an important question:

Why can AISI 1045 reach the required hardness, but still be difficult to control within such a narrow hardness range?

Understanding 45# / S45C / AISI 1045

45# steel, S45C and AISI 1045 are commonly compared as medium-carbon steels used for shafts, rings, mechanical components and other industrial parts.

AISI 1045 typically contains approximately 0.42–0.50% carbon. It can be hardened and tempered, but its through-hardening capability is limited compared with alloy steels with higher hardenability.

This distinction is important.

There is a significant difference between:

“The material can achieve approximately 40 HRC.”

and:

“Every required measuring point can consistently remain between 38 and 42 HRC.”

A 38–42 HRC specification provides only a 4 HRC acceptance window.

For a medium-carbon steel with limited hardenability, component geometry, section thickness and cooling conditions can therefore become important factors.

What Did We Investigate?

After finding the 35–43 HRC variation, we reviewed the heat-treatment process and focused on several possible influencing factors:

*Raw-material composition and material batch

*Component cross-section and geometry

*Furnace temperature uniformity

*Loading arrangement inside the furnace

*Austenitizing and holding conditions

*Quenching medium and cooling rate

*Tempering temperature and uniformity

*Hardness measuring position and method

No obvious process abnormality was identified during our initial investigation.

This led us to look more closely at the relationship between material hardenability, component geometry and the customer's narrow hardness tolerance.

The lesson was simple but important:

Achieving a hardness value is not the same as controlling hardness consistently within a narrow range.

This is something that must be considered during technical review—not after mass production.

Three Heat-Treatment Methods We Should Understand

This project also gave our team an opportunity to review three commonly discussed hardening processes.

1. Through Hardening + Tempering

The basic process is:

Austenitizing → Quenching → Tempering

The objective is to develop the required mechanical properties through the component rather than hardening only a selected surface.

For AISI 1045, published technical information describes hardening by heating into the austenitizing range followed by quenching, with subsequent tempering according to the required properties. However, AISI 1045 has limited through-hardening capability, meaning the final result is influenced by component section size and quenching conditions.

For our caster ring, this was the key issue we needed to consider.

When the customer specifies:

Material: 45# / S45C / AISI 1045

Heat Treatment: Through Hardened & Tempered

Hardness: 38–42 HRC

we should not evaluate the hardness requirement independently from the component geometry.

2. Induction Hardening

Induction hardening works differently.

An alternating electromagnetic field rapidly heats a selected area of the component above its transformation temperature, followed by immediate quenching. The objective is to create a hardened surface layer while leaving the core comparatively unaffected.

The simplified result is:

Surface → Hard, wear-resistant layer

Core → Relatively softer and tougher

This makes induction hardening particularly useful when only a specific working surface requires high hardness and wear resistance.

Typical applications include:

shafts, gears, axles, spindles and other localized wear surfaces.

AISI 1045 is also well suited to flame or induction surface hardening; published data indicates that considerably higher surface hardness can be achieved than the 38–42 HRC required in our present through-hardening project.

Therefore, for some rings, rollers or caster components, an important engineering question is:

Does the customer really require the entire component to have the same hardness, or does only the working surface need to be hardened?

The answer can lead to a completely different heat-treatment solution.

3. Carburizing + Quenching

Carburizing follows another principle.

The simplified process is:

Low-carbon steel → Carbon enrichment of the surface → Quenching → Tempering

Unlike conventional through hardening or induction hardening, carburizing is a thermochemical diffusion process that increases the carbon content near the surface.

After quenching, the objective is to obtain:

Surface → High hardness and good wear resistance

Core → Lower hardness with good toughness

Atmospheric carburizing is widely used for low-carbon steels and is especially useful when a component requires a hard, wear-resistant case combined with a tougher core.

Typical applications include gears, shafts and other heavily loaded components.

For a medium-carbon material such as AISI 1045, however, carburizing would normally not be our first solution simply to achieve a 38–42 HRC requirement. The material already contains sufficient carbon for direct hardening, so the required performance and component design should first be evaluated before changing the heat-treatment route.

What We Learned from This Project

This project reminded our engineering and sales teams of an important principle:

Material + Geometry + Heat Treatment + Hardness Requirement must always be evaluated together.

When receiving a customized component drawing, it is not enough to ask:

“Can 45# steel reach 40 HRC?”

We should also ask:

Where is the hardness measured?

Is the requirement for surface hardness or through hardness?

What is the component section thickness?

Is a case-hardening depth required?

What hardness variation is acceptable between measuring points?

Does the application require wear resistance, core toughness, or both?

For future customized projects with narrow hardness tolerances, our approach will be more cautious:

Review first. Verify when necessary. Then commit.

Sometimes a production challenge is also an opportunity to improve our engineering knowledge.

At CIXI HOTO, we don't want to simply manufacture according to a drawing. We want to better understand why a requirement exists, whether the selected material and process are appropriate, and how to achieve a more stable result in production.

CIXI HOTO — From Components to Solutions.