What Is a Q&T Ground Bar? Hardness by Grade

What does Q&T mean on a ground bar?

Q&T means the bar is quenched and tempered to a hardness band, and ground means the outside diameter, straightness and finish are finished by grinding afterwards. The two steps answer two different questions: heat treatment sets the hardness and strength level through the bar, grinding sets the size and surface your shaft or guide actually runs on.

On the 45 Steel Q&T Ground Round Bar the surface hardness is the guaranteed figure, HRC25–28 or HRC28–32 chosen per order. The core is softer, because grade 45 has limited hardenability — so the band should not be read as a through hardness. This is the point that most often causes a mismatch between drawing and delivery: a buyer writes HRC28–32 expecting the whole section, and a hardness check in the middle of a cut face reads lower.

The 40Cr Q&T Ground Round Bar is specified at Q&T hardness HRC28–35, and the 42CrMo Q&T Chrome-Plated Bar at HRC30–38. In every case the figure is a Q&T hardness band confirmed on the drawing and order, not a case depth.

Because there is no hardened case, a Q&T ground bar suits parts where you want a straight, precise, moderately hard bar with a ground surface — shafting, guide and transmission parts — rather than a wear surface of the kind an induction-hardened shaft provides.

What hardness should I specify for each grade?

Specify the band that belongs to the grade and product you are ordering, and write the product name next to it. The table below gives the bands published by Yixin Precision Machinery (Changzhou).

Grade 45 gives two bands, and you choose one at enquiry. Grade 45 is also the grade where the hardness gradient matters most: the published range describes the surface, and the softer core is a consequence of the grade's limited hardenability, not a deviation.

Foreign designations are useful for orientation only. Grade 45 is comparable, for reference only, to C45, S45C and SAE 1045; 40Cr to 41Cr4 and SCr440; 42CrMo to 42CrMo4 and SCM440. Chemistry limits differ between standards, so a comparable designation is not a substitute for the GB grade on the drawing.

Is a Q&T bar the same as an induction-hardened shaft?

No. In the Yixin range only the 45 Steel Induction-Hardened Chrome Shaft and the 45 Steel Chrome-Plated Linear Shaft carry an induction-hardened case, at a case hardness of at least HRC55 and about 2 mm deep, under hard chrome. Q&T products are quenched and tempered, not surface hardened, and have no induction-hardened case.

The practical split is by duty:

• Choose a Q&T ground bar when you need a straight, ground, dimensionally precise bar at moderate hardness, and when the part is a shaft, guide or transmission element rather than a wear surface.
• Choose an induction-hardened chrome shaft or a linear shaft when the running surface itself must resist wear and the load is carried at the surface.
• Choose the 42CrMo Q&T Chrome-Plated Bar for heavy or shock loads on heavy-duty hydraulic and moving components, where the higher Q&T band and the chrome layer are both wanted. It is made to order, minimum 1 t, lead time 7–10 days, Ø8–100 mm.

If your drawing says case depth but the application only needs a straight ground bar, you are paying for a heat treatment you may not need. If it says Q&T and the part runs under a lip seal or a linear bearing under load, check whether the surface hardness is sufficient.

Which diameter and tolerance apply to Q&T ground bars?

Q&T bars start at Ø8 mm, except the 40Cr Q&T Ground Round Bar, which starts at Ø5 mm. The 45 Steel Q&T Ground Round Bar runs Ø8–140 mm and the 40Cr Q&T Ground Round Bar Ø5–140 mm, both within the standard range of Ø4–140 mm, with up to Ø200 mm made to order for the products where that applies.

For small sizes the band is −0.02/−0.05 mm: Ø8–11 mm for grade 45 Q&T bars and Ø5–11 mm for the 40Cr Q&T Ground Round Bar. From Ø12 mm you have a choice of 0/−0.02, −0.01/−0.03 or −0.02/−0.05 mm; other bands are reviewed against the drawing.

Surface and straightness follow the product: ground bars are specified to Ra ≤ 0.8 μm; chrome-plated bars to Ra ≤ 0.4 μm, with hard chrome 10–50 μm per side chosen per order. Straightness is about 0.1 mm/m from Ø25 mm and about 0.2 mm/m below that. Bars come in full lengths of 4 m, 6 m, 7.5 m or 8.1 m, and can be CNC cut to length from 200 mm with ends left as sawn.

How do I verify hardness and surface quality?

Ask for the heat certificate, and decide at enquiry whether you want third-party inspection arranged for the order. Routine checks at the plant cover diameter, chrome thickness and surface before packing, using Mitutoyo micrometers, a coating thickness gauge, a Rockwell hardness tester and a handheld XRF spectrometer for material grade verification (alloy elements; it does not measure carbon).

Third-party reports on Q&T ground bar samples tested in July 2026 are published at https://yxrod.com/en/quality, and each carries a QR code for verification. On the 45 steel Q&T ground bar sample, Jiangsu Third Party Measurement Technology (TPM, CNAS L11694) measured Ra 0.384 μm and roundness 0.004–0.006 mm; on the 40Cr Q&T ground bar sample, Ra 0.284 μm and roundness 0.001–0.005 mm. The results are in reports TPM-TR 2607015-1 / -2 and apply to the samples tested only. They are one-off third-party checks, not a per-shipment routine; per-order third-party inspection is arranged on request.

A simple inspection plan for an incoming lot:

1. Check the steel mill certificate against the grade ordered, heat by heat.
2. Measure diameter at several points along the bar with a micrometer.
3. Check hardness with a Rockwell tester on the surface, and record where the reading was taken.
4. Check straightness over the bar length, and Ra if the surface is functional.
5. For chrome-plated bars, check chrome thickness and visual surface.

What to put in your RFQ

• Product name and grade with standard, for example 45 Steel Q&T Ground Round Bar, Grade 45 to GB/T 699-2015.
• Diameter and tolerance, for example Ø20 mm, 0/−0.02 mm.
• Hardness band: HRC25–28 or HRC28–32 for grade 45; HRC28–35 for 40Cr; HRC30–38 for 42CrMo.
• Surface: ground only, or Q&T base with hard chrome.
• Length: full bar length, or cut-to-length with sawn ends.
• Straightness and Ra requirements, and any roundness check you plan to run.
• Quantity and delivery term: minimum 1 t per order with sizes combinable, FOB Shanghai or Ningbo.
• Inspection: heat certificate, and whether third-party inspection should be arranged on request.

Summary

A Q&T ground bar is quenched and tempered to a hardness band and then ground for diameter, straightness and finish; it has no induction-hardened case. Specify hardness by grade: grade 45 at surface HRC25–28 or HRC28–32, 40Cr at Q&T hardness HRC28–35, 42CrMo at HRC30–38. Specify diameter and tolerance together with the product name, since Q&T bars start at Ø8 mm and the 40Cr Q&T Ground Round Bar at Ø5 mm; the small band is −0.02/−0.05 mm, and from Ø12 mm you have a choice of 0/−0.02, −0.01/−0.03 or −0.02/−0.05 mm, with other bands reviewed against the drawing. Close the loop with the heat certificate and, where needed, third-party inspection arranged on request.

References

• GB/T 699-2015 — the standard to which Yixin specifies Grade 45.
• GB/T 3077-2015 — the standard to which Yixin specifies 40Cr and 42CrMo.