5160 vs 9260 vs 6150: Choosing Spring Steel Round Bar by Section and Duty

Three spring steels come up in almost every enquiry for suspension and spring stock: AISI 5160, AISI 9260 and AISI 6150. They are all medium-to-high carbon steels that are quenched and tempered to a working hardness, and on a drawing they are often treated as interchangeable. They are not. The differences sit in three alloying elements — silicon, chromium and vanadium — and those differences decide how thick a section you can through-harden, how much the spring resists sag at temperature, and how much you pay per tonne.

This article compares the three grades on chemistry, hardenability, tempering response, delivery condition and cost, and gives a straight answer on which grade suits leaf springs, coil springs, torsion bars and shafts. All composition ranges quoted below come from ASTM A29/A29M, EN 10089 and JIS G4801; property bands are the published delivery-condition and quench-and-temper values from EN 10089. Nothing here is estimated.

What the Three Grades Have in Common

Before the differences, the shared ground matters, because it explains why these grades are grouped at all.

• Carbon in the 0.46–0.64 % band — high enough to reach roughly 58–60 HRC as-quenched, which is what gives a spring its elastic limit, but still low enough to be tempered back to a tough condition instead of a brittle one.

• Supplied annealed or quenched and tempered — annealed bar (+A, typically ≤ 248 HBW on EN 10089 grades) is what you buy if you still have to coil, form or machine. Quenched and tempered bar (+QT) is what you buy when the forming is already done.

• Form first, harden second — with the exception of warm-coiled stock, all three are shaped in the soft condition and only then austenitised and quenched. That is the opposite of a through-hardened shaft grade such as 4140 or 34CrNiMo6, where the bar is usually bought already hardened.

• Fatigue life is a surface property — the final hardening cycle sets the bulk strength, but the surface condition decides when the part fails. Shot peening and decarburisation control matter as much as the grade name on the certificate.

Chemical Composition Compared

The three grades sit in the same carbon band and separate on silicon, chromium and vanadium.

The same three grades appear in the European spring steel standard, EN 10089, under different names. The ranges are not identical to the ASTM ones, and that matters when a customer's drawing calls up the European designation.

What Each Element Actually Does

Reading the two tables side by side answers most selection questions, because each element has one job.

• Silicon, 1.80–2.20 % in 9260 — this is the whole reason 9260 exists. Silicon raises the temperature at which the tempered structure softens, so a 9260 spring holds its load at 200–250 °C where a chromium spring steel of the same hardness has already started to relax. It also raises the elastic limit, which is why silicon-killed spring grades can be run closer to their yield point without taking a permanent set.

• Chromium, 0.70–0.90 % in 5160 and 0.80–1.10 % in 6150 — chromium is the hardenability element. It pushes the continuous-cooling curve to the right so that heavier sections through-harden in oil instead of coming out with a soft ferrite-and-pearlite core. It also adds a little wear resistance, which matters on a leaf spring that sees grit between the leaves.

• Vanadium, 0.15 % min in 6150 — vanadium forms fine carbides that pin the austenite grain boundaries. In practice this means a 6150 bar tolerates a slightly wider furnace window than a plain chromium grade before the grain coarsens, and coarser grain is a direct loss of fatigue life. This is the main technical reason 6150 costs more than 5160 for nearly the same strength.

• Manganese, around 0.7–1.0 % — present in all three, partly as a deoxidiser and partly as a mild hardenability addition. It also ties up sulfur, which is why sulfur is allowed to sit higher (0.040 max) in the ASTM grades than in the EN grades (0.025 max).

Hardenability: the Section Size Decides the Grade

The most common selection mistake is picking a grade on tensile strength and then finding that the middle of the bar did not harden. A spring only works if the whole section transforms to martensite on quenching; a soft core means the surface layer carries the load alone, and the part fails by fatigue from the inside out. Because 9260 has no chromium addition, it has the lowest hardenability of the three. 5160 and 6150 both contain roughly 0.7–1.2 % chromium and through-harden considerably deeper in oil.

That gives a practical rule for round bar: thin sections and wire can use 9260; heavy sections should move to a chromium grade. As a working guide, 9260 is comfortable up to about 30 mm diameter in oil, while 5160 and 6150 will carry comparable sections appreciably thicker before the core starts to go soft. If your part is a 60 mm torsion bar or a thick leaf, chromium is not a preference — it is a requirement. Where the section is large and the duty is severe, the manganese-chromium grade 55CrMnA (SUP9 / 55Cr3) is the next step up and is supplied specifically for thick leaf and heavy coil springs.

Mechanical Properties After Quench and Temper

Spring steels are never specified on the as-rolled condition. The numbers that matter are the annealed hardness — which fixes how easily you can form the part — and the strength band after quench and temper, which is set by the tempering temperature you choose.

Two details in that table are worth reading twice. First, the +QT bands are minimum requirements tied to a section size and a tempering temperature — a 6150 bar tempered at 430 °C and one tempered at 510 °C will both pass the same certificate yet behave differently in a fatigue rig. Second, note where 5160's band sits: 1450–1750 MPa at a minimum 1300 MPa proof is the highest of the three, which is exactly why the chromium-bearing 5160 has been the default for heavy leaf springs for decades.

Equivalent Grades Across Standards

Cross-references are a starting point for a quotation, not a substitution approval. The carbon and chromium bands do not line up exactly, and the hardenability bands (H and HH) are specified separately.

Because the equivalents are close but not identical, a drawing that says "SUP10 or equivalent" should be quoted against 6150 or 51CrV4 — the three share the chromium-vanadium design, and a 50CrVA certificate will usually be accepted. The same logic applies to SUP9A, which maps to 5160. Both the AISI 5160 / SUP9A / 60Cr3 and SUP10 / 6150 / 51CrV4 round bars are stocked with the corresponding EN and JIS designations on the certificate.

Which Grade for Which Part

Matching the grade to the component is mostly a question of section size, temperature and how the spring is loaded.

• Heavy leaf springs, parabolic springs, truck and trailer suspension — 5160 for the standard case, 55CrMnA/SUP9 for very thick leaves. The chromium gives the depth of hardening that a thick leaf needs, and the grade is forgiving in the forge.

• Springs that run warm, or where load loss is the failure mode — 9260. Railway suspension, clutch springs and brake springs that sit at 200 °C and must not sag. Silicon is the element that buys that temperature stability.

• Coil springs, stabiliser bars and torsion bars in medium to heavy sections — 5160 or 6150. If the section is thick and the fatigue budget tight, the vanadium in 6150 pays for itself through finer grain and a wider heat-treat window.

• Valve springs, piston springs and safety-valve springs below 300 °C — 6150 / 51CrV4 / SUP10. These are the classic chromium-vanadium applications and the grade is standardised for them.

• Shafts that are also springs — torsion bars and anti-roll bars are essentially shafts loaded in torsion, so the same rules apply. A shaft that must carry bending load through the whole section without a spring duty is usually better served by a quench-and-tempered alloy steel such as 4140.

Heat Treatment Practice

All three grades are formed soft and hardened afterwards. The quench is the step that decides whether the part survives, and the temper is the step that decides whether it lasts.

Watch the 300 °C region. These grades are all susceptible to temper embrittlement if they are held in the roughly 250–400 °C band, so a tempering temperature is chosen above or below it rather than inside it. A spring is almost never tempered in that window anyway, but a stress-relief cycle after welding or straightening can land there by accident.

Decarburisation is the quiet killer. At 830–880 °C in an atmosphere furnace, a spring steel will lose carbon at the surface within minutes if the atmosphere is not controlled. A decarburised layer is soft, and a spring fails at the surface. Either specify a controlled-atmosphere or vacuum furnace, or machine/grind the decarburised layer off before service, and specify the permissible decarburisation depth on the purchase order.

Shot peen after tempering, not before. Peening puts the surface into compression and is often the single largest fatigue-life gain available on a finished spring — but it has to be the last mechanical operation before coating, and the intensity has to be specified (Almen) rather than left to the shop.

What to Put on the Purchase Order

A grade name alone leaves too much open, and that is where most disputes originate. A workable spring steel order specifies, at minimum:

• Grade and standard — for example "AISI 5160 to ASTM A29" rather than "5160", so the composition limits and the tolerances are unambiguous.

• Hardenability band — plain (H) or restricted (HH). If the part is thick and the quench is marginal, restricting the band is cheaper than scrapping a heat.

• Delivery condition — +A annealed, +AC spheroidised, or +QT with the required tempering temperature range and hardness.

• Decarburisation limit and surface condition — black, peeled, ground or turned, with the permissible decarburisation depth stated.

• Inspection documents — EN 10204 3.1 as a minimum; 3.2 where a third party has to witness the mechanical tests. Straightness, end condition and length tolerance also belong here because they decide how much material you scrap in the coiling cell.

All three grades ship with a heat-specific EN 10204 3.1 certificate giving actual chemistry and mechanical results, not catalogue typicals. Where a standard permits a range, the certificate tells you where in that range your heat actually sits.

Frequently Asked Questions

Is 5160 the same as SUP9A?

Close, but not interchangeable on paper. 5160 to ASTM A29 and SUP9A to JIS G4801 share the same chromium-manganese design with carbon in the 0.56–0.64 % band, and EN 10089 60Cr3 is the European equivalent. The individual limits differ in the third decimal — for example the JIS grade allows 0.70–1.00 % chromium where ASTM allows 0.70–0.90 % — and the sulfur limit is tighter in the JIS and EN grades. For most leaf and coil spring work the substitution is routine, but where the part is safety-critical, buy to the standard named on the drawing and confirm the heat analysis on the certificate.

Why does 9260 resist sagging better than 5160?

Because of the silicon. At 1.80–2.20 % silicon, the tempered carbides in 9260 are more resistant to coarsening, so the structure holds its hardness to a higher temperature. A spring that operates at 200–250 °C will lose load more slowly in 9260 than in a chromium grade of the same initial hardness. The trade is hardenability: no chromium means 9260 cannot through-harden the thick sections that 5160 handles comfortably. That is why heavy leaf springs usually go the chromium route and hot-running springs of moderate section often do not.

Should I choose 5160 or 6150 for a 60 mm torsion bar?

Both will harden at that diameter in oil, so the choice comes down to fatigue budget and process control. 6150 carries more chromium and a vanadium addition, which gives it finer grain and a wider tolerance to furnace variation; 5160 is cheaper for equivalent strength. If the bar has a generous safety factor and your heat treatment is well controlled, 5160 is the economical answer. If the bar is fatigue-critical and the quench is marginal, 6150 is the safer specification, and the vanadium is what you are paying for.

Can these grades be welded?

They can be welded, but a weld is a hard, brittle, crack-sensitive feature in a part that is designed to flex. In practice none of the three is welded in spring service. If a spring has to be joined, preheat in the 250–300 °C range, use a matching or austenitic filler, and temper immediately after welding to relieve the martensite — and even then, expect the weld to be the first thing to fail. If the joint carries the spring load, redesign the part.

What delivery condition should I order?

Order annealed (+A) or spheroidised (+AC) if you still have to coil, roll, cut threads or machine the part — spheroidised anneal is the better choice when there is significant cold forming, because the spheroidal carbides are far softer to deform than lamellar pearlite. Order quenched and tempered (+QT) only when the geometry is already finished and the only remaining operation is a light grind or a shot peen. Buying +QT bar and then reheating it to form the spring wastes both the alloy and the heat treatment you paid for.

Related reading: AISI 9260 / 60Si2MnA / SUP7 spring steel round bar and our comparison of 34CrNiMo6 vs 4140 for heavy shafts.

Request a Quote

Send the grade, the standard, the diameter and length, the delivery condition and the destination port. You will get a quotation with the technical proposal and the available heat analysis within 24 hours. Email enquiry@qilumetal.com.