A quenched and tempered chrome bar and an induction hardened chrome bar differ in where the hardness sits, not in whether the bar is hard. Induction hardening produces a shallow hardened case; quenching and tempering treats the bar through the section at a lower hardness. Both can be finished with hard chrome. The right route depends on where the wear and the load sit.
What actually differs between a Q&T chrome bar and an induction hardened chrome bar?
What differs is depth. Induction hardening hardens a shallow case on the outside of the bar; quenching and tempering changes the bar through the section at a lower hardness and leaves no hardened case at all.
Yixin Precision Machinery (Changzhou) builds two products on the induction-hardened route: the 45 Steel Induction-Hardened Chrome Shaft and the 45 Steel Chrome-Plated Linear Shaft. Both carry a case of ≥ HRC55, about 2 mm deep, formed before hard chrome is applied. The quenched and tempered products are the 45 Steel Q&T Chrome-Plated Bar, the 40Cr Q&T Chrome-Plated Bar, the 45 Steel Q&T Ground Round Bar, the 40Cr Q&T Ground Round Bar, and the 42CrMo Q&T Chrome-Plated Bar; 42CrMo is also available as Q&T ground bar. 42CrMo is supplied in the Q&T condition only, with a minimum order of 1 t and a lead time of 7–10 days. These are quenched and tempered through the section and have no induction-hardened case. For grade 45 the Q&T figure is a surface hardness of HRC25–28 or HRC28–32, chosen per order, and the core stays softer because grade 45 has limited hardenability. For 40Cr the Q&T hardness is HRC28–35. For 42CrMo the Q&T hardness is HRC30–38.
| Item | Induction hardened route | Q&T route |
|---|---|---|
| Products | 45 Steel Induction-Hardened Chrome Shaft; 45 Steel Chrome-Plated Linear Shaft | 45 Steel Q&T Chrome-Plated Bar; 40Cr Q&T Chrome-Plated Bar; 45 Steel Q&T Ground Round Bar; 40Cr Q&T Ground Round Bar; 42CrMo Q&T Chrome-Plated Bar; 42CrMo Q&T ground bar |
| Heat treatment | Case hardened by induction, then hard chrome | Quenched and tempered through the section |
| Hardened case | ≥ HRC55, about 2 mm deep | None |
| Hardness figure | Case ≥ HRC55, about 2 mm deep | 45 steel: surface HRC25–28 or HRC28–32, per order; 40Cr: HRC28–35; 42CrMo: HRC30–38 |
The practical consequence is that a case depth figure and a Q&T hardness figure describe different things and are not interchangeable on a drawing.
Which one fits my application?
Choose by machining needs and surface wear requirements, not by product name. Two questions settle most orders: must the finished running surface itself be hardened, or do you need a quenched and tempered base with room for further work?
If the bar becomes a finished shaft and the running surface has to resist wear, the induction-hardened route is the match. It is supplied as the 45 Steel Induction-Hardened Chrome Shaft, or as the 45 Steel Chrome-Plated Linear Shaft where a tolerance band of 0/−0.02 mm is typical for linear bearings. If you need a quenched and tempered base under the chrome — for higher-load guidance and moving parts — the Q&T chrome-plated bars are the match. The 45 Steel Q&T Ground Round Bar is supplied unplated for higher-load shafts, drive parts and finish-machining stock. Where no surface hardening is wanted at all, the 45 Steel Chrome-Plated Soft Shaft is not induction hardened, with a base of about HRC17.
Size filters the list further. The induction-hardened chrome shaft and the linear shaft run Ø4–130 mm. The 45 steel and 40Cr Q&T chrome-plated bars start at Ø8 mm and reach Ø140 mm, and the 40Cr Q&T Chrome-Plated Bar is made to order in the same Ø8–140 mm range. Unplated, the 45 steel Q&T ground bar runs Ø8–140 mm and the 40Cr Q&T ground bar Ø5–140 mm. The 42CrMo Q&T Chrome-Plated Bar is made to order in Ø8–100 mm.
How deep is the hardened case, and how thick is the chrome?
The induction-hardened case is about 2 mm deep at ≥ HRC55. Hard chrome is 10–50 μm per side, chosen per order, with chrome hardness HV800+.
These are separate entries on a drawing, measured on different features, and they are specified independently of the base condition. Case depth belongs to the induction-hardened products only; on a Q&T bar there is no case to measure, so a case-depth note on a quenched and tempered item has nothing to refer to. Chrome thickness is stated per side, and chrome is applied after grinding and polishing. Put the finished diameter tolerance and the chrome thickness in the same block on the drawing so they are read together rather than in two places.
Hard chrome plating is carried out by long-term partner plants to Yixin's specification, not in Yixin's own shops, and Yixin re-checks chrome thickness, diameter and surface when the bars come back. Salt-spray performance is not part of the standard specification. Where a rating is required, we specify a thicker chrome layer and can have the order tested by a third-party laboratory.
What hardness should I specify, and where do I measure it?
Specify the hardness value and the measurement location, because a surface figure and a case figure are not interchangeable. State whether the number applies to the surface of the bar, to a case at a stated depth, or to the section as a whole.
On a 45 steel Q&T product the figure is a surface hardness: HRC25–28 or HRC28–32, chosen per order. The core of that bar is softer, because grade 45 has limited hardenability, so a through-hardness expectation does not match the supply condition on this grade. On a 40Cr Q&T product the figure is HRC28–35. On the induction-hardened products the specified figure is a case of ≥ HRC55, about 2 mm deep, and that number applies at the case, not to the core.
Grade naming belongs on the drawing as well. Yixin works in Grade 45 to GB/T 699-2015, and 40Cr and 42CrMo to GB/T 3077-2015. Comparable grades from other systems — C45, S45C or SAE 1045 for 45; 41Cr4 or SCr440 for 40Cr; 42CrMo4 or SCM440 for 42CrMo — help an overseas reader orient quickly, but chemistry limits differ between standards, so treat them as comparable, for reference only, rather than identical, and write the grade you want delivered.
How do I check the delivered bar against the order?
Measure the items you specified, using the figures written on the drawing, and confirm the documents that accompany the shipment. Per-order third-party inspection can be arranged on request. The shipping documents are the commercial invoice, packing list and mill certificate, and a certificate of origin can be arranged per order.
| Check | Applies to | Figure to check |
|---|---|---|
| Case hardness and depth | Induction-hardened products | Case ≥ HRC55, about 2 mm deep |
| Surface hardness | 45 steel Q&T products | HRC25–28 or HRC28–32, as ordered |
| Q&T hardness | 40Cr Q&T products | HRC28–35 |
| Chrome thickness | Chrome-plated products | 10–50 μm per side, chosen per order |
| Chrome hardness | Chrome-plated products | HV800+ |
| Diameter | All | Ø4–11 mm: −0.02/−0.05 mm; from Ø12 mm: 0/−0.02, −0.01/−0.03 or −0.02/−0.05 mm |
| Straightness | All | ≈ 0.1 mm/m from Ø25 mm; ≈ 0.2 mm/m below Ø25 mm |
| Surface roughness | All | Ra ≤ 0.4 μm chrome; Ra ≤ 0.8 μm ground |
Note that Yixin does not publish ISO fit codes (f7, g6, h6 …) as its own tolerance promise; bands are quoted as values, and other bands are reviewed against the drawing.
What to put in your RFQ
- Product name and grade, for example 45 Steel Q&T Chrome-Plated Bar or 40Cr Q&T Chrome-Plated Bar, or the 45 Steel Induction-Hardened Chrome Shaft or 45 Steel Chrome-Plated Linear Shaft.
- Hardening route: quenched and tempered, or induction-hardened case, stated explicitly rather than assumed from the product name.
- Hardness value and location: surface hardness for Q&T, or case hardness plus case depth for induction-hardened items.
- Chrome requirement where plated: thickness per side, chrome hardness, and any third-party test to be arranged on request.
- Diameter, tolerance band and length, including cut-to-length requirements; full bars are 4 m, 6 m, 7.5 m or 8.1 m and CNC cutting to length from 200 mm is available.
- Straightness and surface roughness limits, since both are separate from diameter.
- Quantity and delivery terms: minimum 1 t, with 20–50 kg samples; FOB Shanghai or Ningbo, T/T (50% deposit, balance before shipment).
- Documents required with the shipment, and whether per-order third-party inspection should be arranged.
Summary
Quenched and tempered chrome bar and induction hardened chrome bar are not competing grades of the same thing; they place hardness in different places. Induction hardening gives a shallow case of ≥ HRC55, about 2 mm deep, on the 45 Steel Induction-Hardened Chrome Shaft and the 45 Steel Chrome-Plated Linear Shaft. Quenching and tempering treats the bar through the section, giving HRC25–28 or HRC28–32 as a surface figure on 45 steel Q&T products and HRC28–35 on 40Cr Q&T products, with no case. Hard chrome is a separate layer of 10–50 μm per side, chosen per order, at HV800+. Specify the route, the hardness value with its location, the chrome thickness per side, and the tolerance band, then inspect against those same items.
References
- GB/T 699-2015 — Quality carbon structure steels (the standard for Yixin's Grade 45).
- GB/T 3077-2015 — Alloy structure steels (the standard for Yixin's 40Cr and 42CrMo).