Reading a Chinese Shaft Quote: 45#, Soft Shaft, Hard Shaft, Q&T

What does 45# mean on a Chinese shaft quote?

45# identifies the steel grade only. It does not tell you the heat treatment, the hardness, the tolerance or the surface, so treat it as the starting point of the quote rather than the specification.

Grade 45 to GB/T 699-2015 is a Chinese carbon steel. Yixin names its grades with the Chinese grade and standard first, and foreign grades only as comparable, for reference only, because chemistry limits differ between standards: a comparable grade is not a drop-in substitute, and the drawing should state which grade and standard you are buying to. Yixin's catalogue covers 45 steel and 40Cr; 42CrMo is supplied in Q&T condition only, Ø8–100 mm, made to order, with a minimum order of 1 t. 40Cr and 42CrMo are supplied to GB/T 3077-2015. The same grade can be quoted in several conditions, so the grade line alone never fixes the part. What changes the part is what happens after the bar is drawn or peeled: whether the surface is induction hardened, whether the bar is quenched and tempered, and whether chrome is applied. Those are the lines that decide hardness, machinability and wear behavior. When a quote lists only 45# and a diameter, ask for the condition and the surface before comparing it with another supplier's offer.

Soft shaft, hard shaft or Q&T — which condition is on the quote?

The condition line decides whether the bar has a hard case, a quenched-and-tempered base, or neither. Soft shaft and hard shaft are the same Grade 45 steel taken through different process routes — not one product in two finishes.

A soft shaft is a chrome-plated shaft whose surface is not induction hardened. Base hardness is approximately HRC17, and the route suits shafts that still need turning, milling or drilling, plus support and guidance duties and general sliding fits. A hard shaft, in quoting practice, usually means the induction-hardened chrome shaft: 45 steel, induction hardened and then hard chrome plated, with a case of at least HRC55, approximately 2 mm deep, used for guide pillars and guidance, hydraulic cylinders and moving parts, and wear-resistant sliding fits. Q&T is a third condition. Q&T products are quenched and tempered but have no induction-hardened case: 45 Steel Q&T Ground Round Bar is induction quenched and tempered to a surface hardness of HRC25–28 or HRC28–32, chosen per order (the core is softer), and left ground, while 40Cr Q&T Ground Round Bar reaches HRC28–35. A chrome layer can be added to a Q&T bar as well, producing 45 Steel Q&T Chrome-Plated Bar or 40Cr Q&T Chrome-Plated Bar, the latter made to order.

How do I read the tolerance and straightness callouts?

Size tolerance and straightness are two different requirements, and one number cannot cover both. Size tolerance controls a size range; straightness controls form.

Yixin's published tolerance bands are −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 upward. For a linear shaft, 0/−0.02 mm is typical for linear bearings. Straightness is a separate figure: approximately 0.1 mm/m from Ø25 mm, and approximately 0.2 mm/m below that. Surface finish follows the surface: Ra ≤ 0.4 μm for chrome, Ra ≤ 0.8 μm for ground bar. Diameter availability runs from Ø4–140 mm from stock, with up to Ø200 mm made to order, product-dependent (induction-hardened and linear shafts Ø4–130 mm; Q&T bars from Ø8 mm, 40Cr Q&T ground bar from Ø5 mm), and full bars come in 4 m, 6 m, 7.5 m or 8.1 m, with CNC cutting to length from 200 mm. All of these are standard capabilities confirmed on the drawing and order, not values that apply automatically to every line of a quotation. When you compare two offers, check that the tolerance is attached to the same diameter and the same surface condition; a tolerance quoted on a ground bar and a tolerance quoted on a chrome-plated bar are being measured on different surfaces.

What does the hard chrome layer actually specify?

Chrome is specified as thickness per side plus coating hardness, not as a total layer and not as a color. Yixin's hard chrome layer is 10–50 μm per side, chosen per order, and chrome hardness is HV800+.

Hard chrome plating is done by long-term partner plants to Yixin's specification, and Yixin re-checks chrome thickness, diameter and surface when the bars come back. Two consequences matter when you read a quote. First, chrome does not change the hardness of the base: if the application needs a hard case, that comes from induction hardening, and if it needs a quenched-and-tempered base, that comes from Q&T. Second, the plated diameter is a fit dimension, so state whether the tolerance applies before or after plating. 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. Per-order third-party inspection is arranged on request and should be agreed before the order rather than assumed from the quote.

Why does the quote list so many process steps?

Each step in the route is a separate operation, and each one is a point where the bar can move out of tolerance. Reading the route tells you which operations are in-house and which are subcontracted.

For the soft shaft the route is cold drawing (≤ Ø28 mm) or peeling (Ø30 mm and above), then roll straightening, centerless grinding, polishing, hard chrome plating at a partner plant and final polishing. The induction-hardened route inserts induction hardening after centerless grinding and before polishing. In-house at Yixin's Changzhou plant, cold drawing, peeling, roll straightening, induction Q&T, induction hardening, centerless grinding, polishing and CNC cutting all run in the workshop; hard chrome plating and machining are handled by partner plants to Yixin's specification and checked on return. Ask which lines on the quotation are in-house and which are partner operations, and which dimension is verified after each one. That question matters more than the number of steps, because it tells you where a tolerance change can be absorbed and where it cannot.

What to put in your RFQ

• Grade and condition: for example Grade 45 soft shaft, Grade 45 induction hardened, or Grade 45 Q&T, and the standard you are buying to.
• Diameter and tolerance band, tied to the diameter you actually measure: −0.02/−0.05 mm for Ø4–11 mm, or 0/−0.02 mm, −0.01/−0.03 mm, −0.02/−0.05 mm from Ø12 mm upward.
• Straightness and surface finish if the application depends on them, stated separately from the size tolerance.
• Chrome requirement: which surfaces are plated, thickness per side, and whether the quoted diameter is measured before or after plating.
• Hardness requirement: surface hardness for 45 steel Q&T (HRC25–28 or HRC28–32), the Q&T range for 40Cr or 42CrMo, or induction-hardened case hardness and depth, and the area where it must hold.
• Length: full bars (4 m, 6 m, 7.5 m or 8.1 m) or CNC cut to length, with the cut tolerance and end condition.
• Quantity and delivery basis: production orders from 1 t, samples 20–50 kg, FOB Shanghai or Ningbo, T/T (50% deposit, balance before shipment).
• Acceptance items and reports, agreed before the order, including whether per-order third-party inspection is required.

Summary

A 45# quote has four readable lines: grade, material condition, tolerance and straightness, and surface. The grade tells you the steel only. The condition tells you whether there is an induction-hardened case (Induction-Hardened Chrome Shaft and Linear Shaft), a quenched-and-tempered base with no case (Q&T series), or neither (soft shaft and ground bar). Tolerance and straightness are separate requirements, and chrome is specified as thickness per side at a coating hardness, applied by partner plants to specification. When the four lines are complete and the acceptance items are agreed before the order, two quotations can be compared on the same basis.

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).