Ground Bar vs Chrome Bar vs Q&T Ground Bar for Machined Shafts

What is the difference between ground bar and chrome bar?

Ground bar and chrome bar can share the same steel and much of the same preparation; the difference is whether a hard chrome layer is applied to the surface.

Yixin Precision Machinery (Changzhou) supplies the 45 Steel Ground Round Bar in 45 steel, not Q&T, at approximately HRC17 with a ground surface of Ra ≤ 0.8 μm. The 45 Steel Chrome-Plated Soft Shaft uses the same unhardened base and then receives hard chrome of 10–50 μm per side, chosen per order, at a chrome hardness of HV800+ and a finish of Ra ≤ 0.4 μm. Neither product has an induction-hardened case, so the chrome layer is the difference that matters where the bar meets a seal, a guide or a bearing.

The practical test is where the surface of the finished part has to work. If the supplied diameter is already the working diameter, the chrome surface is the one in service. If the bar is an input to turning, milling, threading or keyway cutting, the ground surface is the one to order.

Should you buy a ground bar or a chrome bar for a shaft you will machine?

Buy a ground bar or a Q&T ground bar when substantial machining happens after supply, and treat chrome as a final surface. A plated bar cannot be turned or milled on the plated length without removing the chrome from the machined zone.

Start with load, base material, surface requirement and the replacement drawing, and do not infer material, hardness or chrome from the old part name alone. Where substantial machining follows supply, the routes to evaluate first are a chrome-plated soft shaft, a ground bar or a Q&T ground bar. A chrome-plated soft shaft keeps chrome on the working surface over an unhardened base, so the plated length can stay uncut while soft ends take threads or other features.

Where the supplied diameter and the plated surface are the finished working surface, a chrome bar is the correct purchase. Where the part is still being formed, a ground bar is the correct purchase. The mismatch does not show up in the bar; it shows up in the process after it.

When does a Q&T ground bar make sense instead of a plain ground bar?

A Q&T ground bar makes sense when the shaft needs more than the soft base condition but still needs machining allowance. Quenched and tempered bar is heat treated in the Q&T condition and then ground; it is not surface hardened and has no induction-hardened case, so it is a base-condition upgrade rather than a wear surface.

The 45 Steel Q&T Ground Round Bar is 45 steel in the induction Q&T condition with a surface hardness of HRC25–28 or HRC28–32 chosen per order; the core is softer because grade 45 has limited hardenability. Where a higher Q&T hardness is wanted, the 40Cr Q&T Ground Round Bar is supplied in 40Cr at HRC28–35. Both are ground bars, and the matching chrome variants, the 45 Steel Q&T Chrome-Plated Bar and the 40Cr Q&T Chrome-Plated Bar, add the chrome working surface when the part is close to final. Where the finished shaft carries heavy or shock loads, the 42CrMo Q&T Chrome-Plated Bar is quenched and tempered to HRC30–38 and then hard chromed, made to order from 1 t with a 7–10 day lead time.

On the drawing, write the 45 steel Q&T hardness as HRC25–28 or HRC28–32 per order rather than as a core or through-hardness value, because the specified range applies to the surface and the core is softer.

How thick should the chrome be, and which tolerance and finish should you state?

State chrome thickness, diameter tolerance, straightness and surface finish on the drawing, because these are confirmed on the order. For Yixin chrome bars, hard chrome is 10–50 μm per side, chosen per order, with chrome hardness HV800+, a chrome finish of Ra ≤ 0.4 μm and a ground finish of Ra ≤ 0.8 μm.

Diameter tolerance is −0.02/−0.05 mm for Ø4–11 mm, and from Ø12 mm the drawing should name 0/−0.02 mm, −0.01/−0.03 mm or −0.02/−0.05 mm. Q&T bars start at Ø8 mm; the 40Cr Q&T ground bar starts at Ø5 mm. Straightness is approximately 0.1 mm/m from Ø25 mm and approximately 0.2 mm/m below Ø25 mm. Bars are supplied in 4 m, 6 m, 7.5 m or 8.1 m lengths, CNC cut to length from 200 mm with ends left as sawn, so a cut length is an ordered item rather than a stock default.

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 to put in your RFQ

• Product route: name the catalogue product you want, or describe the route and the last machining step, so the supplier can propose a ground bar, a chrome bar, a Q&T ground bar or a Q&T chrome-plated bar.
• Grade and base condition: Grade 45, 40Cr or 42CrMo, and whether Q&T is required. Comparable grades such as C45, S45C or SAE 1045 are for reference only, because chemistry limits differ between standards.
• Diameter and tolerance: nominal diameter with the tolerance class, −0.02/−0.05 mm for Ø4–11 mm and 0/−0.02 mm, −0.01/−0.03 mm or −0.02/−0.05 mm from Ø12 mm.
• Length: full bars of 4 m, 6 m, 7.5 m or 8.1 m, or cut to length from 200 mm with ends left as sawn.
• Hardness: HRC25–28 or HRC28–32 for 45 steel Q&T products, HRC28–35 for 40Cr Q&T, HRC30–38 for 42CrMo Q&T.
• Surface: ground Ra ≤ 0.8 μm or chrome Ra ≤ 0.4 μm, and chrome thickness of 10–50 μm per side where chrome is specified.
• Straightness, and any length or feature that must stay unplated.
• Quantity, packing and terms: minimum 1 t, samples 20–50 kg, export packing, and FOB Shanghai or Ningbo.

Summary

Ground bar, chrome bar and Q&T ground bar answer three different questions. Ground bar is the machining input: 45 steel at approximately HRC17 with Ra ≤ 0.8 μm. Chrome bar is the finished working surface: hard chrome of 10–50 μm per side at HV800+ with Ra ≤ 0.4 μm. Q&T ground bar is the base-condition upgrade: quenched and tempered, not surface hardened, specified as HRC25–28 or HRC28–32 on 45 steel and HRC28–35 on 40Cr. Order the route that matches your last machining step.