Induction Hardening vs Carburizing for Shafts: Which to Specify?

Which route fits a hydraulic piston rod or a guide shaft?

For a piston rod or guide shaft that is finished by grinding and hard chrome plating, induction hardening is normally the practical choice. Carburizing belongs on the drawing when the case is defined as CHD to 550 HV (to be confirmed against the applicable standard).

The reasoning is in what happens after the heat treatment. Induction surface hardening gives good repeatability with minimal distortion and minimal surface oxidation. On a long, slender bar those two effects decide how much work is left after hardening: distortion has to be taken out by straightening or grinding, and oxidation has to be removed before plating.

On a chrome-plated shaft the hardened case sits at the surface, under the chrome, where seals and guides run. Yixin supplies the 45 Steel Induction-Hardened Chrome Shaft, a Grade 45 bar that is induction hardened to a case of at least HRC55 and about 2 mm deep, then polished, hard chrome plated and final polished. The 45 Steel Chrome-Plated Linear Shaft follows the same route, with tolerances set for linear bearings.

Specify induction hardening when the case can be produced at or near the finished diameter. Specify carburizing when your drawing, your customer's specification or your heat treater sets the case in CHD to 550 HV (to be confirmed against the applicable standard); in that case the finishing allowance is planned with the heat treater rather than with the bar supplier alone.

How is case depth defined for induction hardening and carburizing?

The two routes do not share one depth definition. Case hardening depth (CHD) is measured to a 550 HV limit (to be confirmed against the applicable standard). Induction surface hardening depth (SHD) is commonly measured to 0.80 × the minimum surface hardness (to be confirmed against the standard specified on the drawing).

This is the most useful point to settle before comparing quotations, because two suppliers can quote the same number in millimetres and mean different things. A depth written to a lower hardness limit is measured further into the bar than the same layer measured to a higher limit, and the SHD limit moves whenever the specified surface hardness changes. Always write the limit next to the number.

For an induction-hardened chrome shaft, the practical instruction is a case of at least HRC55 with the depth stated as SHD and the hardness limit named, measured under the chrome. For a carburized part, the instruction is case hardening depth to 550 HV (to be confirmed against the applicable standard) with the depth in millimetres.

Where does hardening sit in the process route?

Hardening comes after the bar is ground to size and before polishing and plating; the chrome is applied near the end of the route and only polished afterwards.

Yixin's route for the 45 Steel Induction-Hardened Chrome Shaft runs: cold drawing at Ø28 and below or peeling from Ø30, roll straightening, centerless grinding, induction hardening, polishing, hard chrome plating, final polishing. Hard chrome plating is done by a long-term partner plant to Yixin's specification and re-checked by Yixin on return.

Because induction hardening brings minimal distortion and minimal surface oxidation, the operations after it are polishing rather than heavy stock removal, and the hardened surface is not cut away once the case has been produced. That is also why the case is measured under the chrome rather than on it.

Where it applies: this sequence suits bars whose finished diameter is close to the supplied diameter, which is the usual case for piston rods and guide shafts. If the drawing instead requires a case defined as CHD to 550 HV (to be confirmed against the applicable standard), the heat treatment is followed by the machining that route calls for, and the depth is verified to the CHD definition.

What should the inspection plan cover?

The plan should fix four things: the case depth with its hardness limit, the hardness just below the surface, the chrome layer, and the geometry of the finished bar.

• Case depth stated as SHD or CHD, with the hardness limit used named in the same line.
• Hardness measured below the surface and reported in HV at stated depths.
• Chrome thickness of 10–50 μm per side, chosen per order, and chrome hardness can reach HV800+.
• Roughness of Ra ≤ 0.4 μm on chrome and Ra ≤ 0.8 μm on ground surfaces; straightness of about 0.1 mm/m from Ø25 and about 0.2 mm/m below.

Third-party measurement is the way to close this out. On a July 2026 sample bar, Jiangsu Yinshi Inspection & Testing reported an effective case depth of SHD475 = 1.91 mm, with HV1 757–783 measured 0.1–0.8 mm below the surface. Those results apply only to the sample bars tested. Third-party inspection of a production order is arranged on request. Note that the case depth report names its hardness limit alongside the depth, which is the practice to require in any case depth statement.

Induction hardening, induction Q&T or soft shaft, which am I ordering?

Only the induction-hardened chrome shaft and the linear shaft carry an induction-hardened case. The Q&T bars are quenched and tempered, not surface hardened, and the soft shaft is not hardened at all.

• 45 Steel Induction-Hardened Chrome Shaft and 45 Steel Chrome-Plated Linear Shaft: induction-hardened case of at least HRC55, about 2 mm deep, then hard chrome.
• 45 Steel Q&T Ground Round Bar and 45 Steel Q&T Chrome-Plated Bar: induction quenched and tempered; surface hardness HRC25–28 or HRC28–32, chosen per order. The core is softer because Grade 45 has limited hardenability, so this range is a surface hardness, not a through or core hardness.
• 40Cr Q&T Ground Round Bar and 40Cr Q&T Chrome-Plated Bar: quenched and tempered, HRC28–35.
• 42CrMo Q&T Chrome-Plated Bar: quenched and tempered, HRC30–38, made to order, for heavy or shock loads.
• 45 Steel Chrome-Plated Soft Shaft and 45 Steel Ground Round Bar: not induction hardened, base about HRC17.

Grades follow GB/T 699-2015 for Grade 45 and GB/T 3077-2015 for 40Cr and 42CrMo. Grades such as C45, S45C, SAE 1045, 41Cr4, SCr440, 42CrMo4 and SCM440 are comparable for reference only; chemistry limits differ between standards.

What to put in your RFQ

• The product and route you want, for example a 45 steel induction-hardened chrome shaft, or a Q&T chrome-plated bar where no case is required.
• Case depth in mm and the limit it is measured against: SHD at 0.80 × minimum surface hardness (to be confirmed against the standard specified on the drawing), or CHD at 550 HV (to be confirmed against the applicable standard).
• Minimum surface hardness and the depth at which it must be met; for the induction-hardened chrome shaft this is a case of at least HRC55.
• Chrome thickness per side, 10–50 μm chosen per order, and chrome hardness can reach HV800+.
• Roughness and straightness limits, and whether each applies to the chrome or to the ground surface.
• Diameter with tolerance, and the delivery length and cut length you need.
• The heat treatment condition of the base bar, so the order is not placed as a soft shaft by mistake.
• Whether a third-party case depth or roughness report is required; inspection of an order is arranged on request.

Summary

The choice comes down to two questions: how the drawing defines the case, and what the bar has to survive after heat treatment. If the case is defined as surface hardening depth at 0.80 × the minimum surface hardness (to be confirmed against the standard specified on the drawing), induction hardening is the route, and it brings minimal distortion and minimal surface oxidation. If the case is defined as case hardening depth to 550 HV (to be confirmed against the applicable standard), that definition governs the order. In both cases, write the depth, the hardness limit, the chrome thickness and the inspection method on the drawing, and confirm the final values on the order.

References

• GB/T 699-2015 — grade standard cited for Grade 45.
• GB/T 3077-2015 — grade standard cited for 40Cr and 42CrMo.