A specification of 2 mm, HRC55 on an induction hardened shaft fixes two things at once: the hardened case in the steel extends about 2 mm below the surface, and the hardness of that case, sitting under the chrome where seals and guides run, is at least HRC55. The 2 mm is a case depth in the steel, not a coating thickness.
What does the 2 mm in a case depth specification measure?
The 2 mm measures how far the hardened layer extends into the steel from the surface downward; it says nothing about the chrome layer on top of it.
Induction hardening heats the surface zone of the bar and quenches it, so the hardening is a layer rather than a through-section change. Because the transition from hard case to softer material underneath is gradual, a case depth cannot be read off directly. It is established by a hardness traverse on a cross-section, and the depth is quoted against a hardness limit chosen for that measurement — the limit at which the case is considered to end. That is why a case depth figure is only meaningful when the limit, the test load and the measurement locations are reported with it.
On the 45 Steel Induction-Hardened Chrome Shaft, the case is at least HRC55 and about 2 mm deep. The depth is a nominal figure for the route; the measured value on any bar depends on the traverse, and the report tells you what was found. A single reading just below the surface is not a case depth: the depth describes the whole hardened layer, so the hardness has to stay above the limit through the stated distance.
Why is the case hardness written as HRC55 and not as a range?
HRC55 is a floor for the hardened case — a minimum acceptance value at the surface beneath the chrome, not a typical or an average figure.
Buyers often ask for a range because ranges feel more precise. In practice a minimum is the useful form for a wear surface: the case either reaches the floor through the specified depth or it does not, and the part is accepted or rejected on that basis. A range with an upper limit would be harder to hold and would say little about performance, since harder is not automatically better for a running surface that has to work with a seal or a bearing.
Two points matter when you compare certificates. First, Rockwell and Vickers are different scales, and comparison is only sound at the same scale, load and location; a converted figure carries conversion limits and should not be treated as a measurement. Second, the location must be under the chrome, not on it. A hardness reading taken on the plated surface describes the chrome, not the case.
Where a drawing needs a Vickers value, state it as a Vickers requirement with the load, and let the case-depth traverse confirm it at the same time.
Is case depth the same as chrome thickness?
No. They are two different layers with two different jobs, and mixing up their units is one of the most common specification errors.
The induction-hardened case is in the steel and is measured in millimeters. The hard chrome is on the steel and is measured in micrometers: 10–50 μm per side, chosen per order. On the 45 Steel Induction-Hardened Chrome Shaft the sequence is induction hardening, then polishing, then hard chrome plating and final polishing, so the hardened case sits under the chrome. Chrome hardness capability reaches HV800+, which is a separate figure again and does not describe the steel beneath.
A drawing that asks for 2 mm of chrome, or for a 2 mm case measured in micrometers, cannot be quoted as written. State both: the case depth with its hardness floor in the steel, and the chrome thickness per side with the final diameter and tolerance the plating has to land on. Because plating adds thickness per side, the chrome figure and the final size have to be read together — the grinder works to the pre-plate size that the chosen layer requires.
Hard chrome plating for Yixin products is carried out by long-term partner plants to Yixin's specification, while forming, roll straightening, centerless grinding and induction hardening are done in the Yixin workshop in Changzhou.
| Layer | Where it sits | How it is stated | What it covers |
|---|---|---|---|
| Induction-hardened case | In the steel, below the chrome | At least HRC55, about 2 mm deep | The running surface under seals and guides |
| Hard chrome | On the steel surface | 10–50 μm per side, chosen per order; hardness capability HV800+ | The outer contact surface |
| Substrate below the case | The body of the bar | Not specified as a case figure | The unhardened remainder of the section |
How is case depth verified, and what does a report show?
Case depth is verified by a hardness traverse on a prepared section, and the report should name the hardness limit, the load and the positions measured.
Yixin publishes a case-depth report on a July 2026 sample bar of the 45 Steel Induction-Hardened Chrome Shaft, issued by Jiangsu Yinshi Inspection & Testing Co., Ltd. The laboratory measured an effective case depth of SHD475 = 1.91 mm, with HV1 757–783 at 0.1–0.8 mm below the surface. That report carries no CMA or CNAS mark, and each report carries a QR code so the figure can be checked with the laboratory. The result applies to the samples tested; it is a sample check, not a batch release.
When you read any case-depth report, check four things: the hardness limit that defines the end of the case, the test load, the depth positions measured, and whether the traverse started below the chrome. Then compare the figure with what your drawing asks for, not with a converted hardness on another scale. If your project needs third-party inspection of a specific order, it is arranged on request and should be written into the enquiry rather than assumed.
| Check | What to look for in the document |
|---|---|
| Hardness limit | The limit defining the end of the case, stated with the depth figure |
| Test load and scale | Stated with the readings; not converted from another scale |
| Measurement position | Locations below the surface, starting under the chrome |
| Chrome thickness | Reported separately, in μm per side |
| Scope | Which sample the result applies to |
Which Yixin products have an induction-hardened case?
Only the 45 Steel Induction-Hardened Chrome Shaft and the 45 Steel Chrome-Plated Linear Shaft carry an induction-hardened case; quenched and tempered products do not.
This distinction decides whether a case depth belongs on your drawing at all. The linear shaft follows the same route as the induction-hardened chrome shaft, with tolerances set for linear bearings. Quenched and tempered bars are a different product family: they are quenched and tempered rather than surface hardened, so a case-depth requirement does not apply to them, and their hardness figures describe the quenched and tempered state.
Grade names follow Chinese standards: Grade 45 to GB/T 699-2015, and 40Cr and 42CrMo to GB/T 3077-2015. Comparable foreign grades such as C45, S45C or SAE 1045 for Grade 45, and 41Cr4 or SCr440 for 40Cr, are for reference only — chemistry limits differ between standards.
| Product | Induction-hardened case | Hardness figure | Diameter range |
|---|---|---|---|
| 45 Steel Induction-Hardened Chrome Shaft | Yes | Case at least HRC55, about 2 mm deep | Ø4–130 mm |
| 45 Steel Chrome-Plated Linear Shaft | Yes, same route | Case at least HRC55, about 2 mm deep | Ø4–130 mm |
| 45 Steel Chrome-Plated Soft Shaft | No | Base about HRC17 | Standard range Ø4–140 mm, up to Ø200 mm made to order |
| 45 Steel Q&T Chrome-Plated Bar | No | Q&T surface hardness HRC25–28 or HRC28–32, chosen per order; the core is softer | Ø8–140 mm |
| 40Cr Q&T Chrome-Plated Bar | No | Q&T hardness HRC28–35 | Ø8–140 mm, made to order |
For 45 steel, the quenched and tempered surface hardness is stated as HRC25–28 or HRC28–32 because the core is softer — grade 45 has limited hardenability — so that figure should never be read as a through or uniform hardness.
What to put in your RFQ
- The product name and grade: 45 Steel Induction-Hardened Chrome Shaft, Grade 45 to GB/T 699-2015, or the matching product if a case is not required.
- Case requirement in two parts: minimum case hardness (for example HRC55) and case depth (for example about 2 mm), with the hardness limit to be used for the effective case depth.
- Chrome thickness per side within 10–50 μm, plus the final diameter and tolerance band the plated bar must meet.
- Diameter and cut length: full bars of 4 m, 6 m, 7.5 m or 8.1 m, or CNC cut to length from 200 mm with ends left as sawn.
- Straightness and surface finish: Ra ≤ 0.4 μm on chrome, together with the straightness your application needs.
- Whether any machining, milling or drilling is planned after supply, so the effect of the hardened case and the chrome can be reviewed before selection.
- Whether a third-party inspection report is required on the order, and which characteristics it should cover.
Summary
A 2 mm, HRC55 callout is two requirements in one line: a hardened case about 2 mm deep in the steel, and a minimum hardness of HRC55 at the surface that seals and guides run on. It is not a coating thickness. The chrome sits on top of the case at 10–50 μm per side, chosen per order, and it is measured on a different scale in different units.
To specify it cleanly, state the case hardness and depth with the hardness limit used to define the end of the case, state the chrome thickness per side with the final diameter and tolerance, and name the product so the requirement lands on a bar that actually has a case. Of the Yixin range, that means the 45 Steel Induction-Hardened Chrome Shaft or the 45 Steel Chrome-Plated Linear Shaft, both Ø4–130 mm; the quenched and tempered bars take a hardness figure instead of a case figure.