Gerdau Heat Treated Bars: Match the Process to the Property You Need

I was standing at the process desk in a machine shop with a drive-shaft drawing in one hand and the incoming mill certificate in the other, and the two were contradicting each other. The callout under the part number read HT 28-32 HRC. The certificate said the material had been heat treated, and that was the whole acceptance line. Nothing told me whether 28-32 was taken on the surface or at the center, and nothing told me which of several possible treatments produced it. The machinist wanted to start cutting. I wanted to know what we had actually bought.

If you are the one writing the purchase order, the treatment name is the least useful field on the page. Gerdau runs six in-house heat treatments, and each one pushes a different property in a different direction, so the same grade of special bar quality steel can leave the mill in states that are not interchangeable. What follows is the order I use to filter a heat treated Gerdau bar from a drawing requirement down to a line item, and the single detail I confirm before the order goes out.

Why "Heat Treated" on a Print Isn't a Specification

When a print says only heat treated, what has the supplier actually been asked to deliver? Gerdau's own framing is that heat treating broadens the spectrum of properties that can be reached with special bar quality steel bars. Read that literally. A single heat of SBQ steel can leave the mill as several different products, and the phrase heat treated names none of them. It says a thermal cycle happened; it does not say what the cycle did.

Turning a vague requirement into something a supplier can be held to means pinning down the property itself before any grade name enters the conversation: the hardness or strength value, the test method behind it, the metallurgical condition it applies to, and the location on the bar where the reading is taken. Leave those open and two suppliers can both meet your print while shipping bars that behave differently in service.

The gap between those readings is metallurgical, not cosmetic. A shallow cycle can harden a bar's surface while the core never transforms; a through-section quench and temper drives the whole section to the same reading. Identical numbers on the certificate, different fatigue life in the part.

Six In-House Processes, Six Different Goals

Gerdau's in-house list runs cold shear annealing, thermal stress relieving, normalizing, lamellar pearlitic annealing, spheroidize annealing, and quench and tempering, and its published goals run improve machinability, increase toughness, improve cold forming characteristics, alter hardness and tensile strength up or down, relieve residual stress, and improve shearability. Which process moves which property, and in which direction?

Spheroidize annealing is the softest route. It takes the layered iron carbide inside the steel and coarsens it into rounded particles, breaking up the hard phase that wears out cutting tools; that is what you specify for high-carbon and bearing-type grades headed for cold forming or heavy machining. Lamellar pearlitic annealing keeps the carbide in its layered form and only softens it part way, so you hold on to more strength while machinability improves. Normalizing heats the bar above its transformation temperature and cools it in still air, refining and evening out the grain; the payoff is uniform hardness and predictable dimensions through a long machining sequence.

Quench and tempering is the only route that raises strength and toughness at the same time. The bar is cooled fast enough to form hard martensite, then reheated to pull hardness back to a controlled level, and the tempering temperature sets the final number. Thermal stress relieving runs below the transformation temperature and changes hardness very little; its job is to relax the residual stress left by straightening, cold drawing, or rough machining so the part does not move after final cuts. Cold shear annealing softens the bar specifically so it cuts cleanly in a shear without cracking.

A Filter Order That Keeps You Out of Trouble

Should you pick the grade first and then hunt for a treatment that fits, or fix the treatment first and let it decide the grade? The order is not cosmetic. Get it wrong and a change late in the project wipes out everything you selected.

Step one is the property and how it will be accepted, because the part's loading decides it. A shaft in torsion wants a through-section hardness and a tensile floor; a fastener that gets crimped cold wants softness and a cold-forming limit. Write the acceptance value, the scale, and the test location now, before any grade name enters the conversation.

Step two is the treatment that produces that property, chosen from Gerdau's six. Match the goal to the process: machinability or cold forming points to spheroidize or lamellar pearlitic annealing; strength with toughness points to quench and temper; dimensional stability points to stress relieving; a uniform structure ahead of heavy machining points to normalizing.

Step three is the grade, and here the grade does less work than most people assume. Chemistry and hardenability set whether a given treatment can actually be carried out across the section you need; a lean grade simply cannot be through-hardened in a large diameter no matter what the certificate promises. Step four is the certificate and the batch traceability. Reversing the order is what hurts. Name the grade first and you are locked into the process route that grade is normally sold under, so when you later realize the part needed machinability rather than strength, you throw away the heat and start again.

Which Process-Grade Pairs Make the Shortlist, and Which Fail

With the order fixed, which combinations belong on a shortlist and which should be struck off? Four pairings cover most of what crosses a machine-shop desk. Quench and tempering with a medium-carbon alloy grade, the 4140 family, is the standard choice for a high-strength shaft that also has to absorb shock. Normalizing with a medium-carbon grade such as 1045 suits a part that will be machined hard and needs even structure and stable dimensions more than peak strength. Spheroidize annealing with a high-carbon or bearing grade like 52100 is what you reach for when the bar has to be cold formed or turned aggressively. Thermal stress relieving suits long bars and precision parts where the enemy is movement after machining.

The exclusions matter just as much. If the only problem you are solving is tool wear, do not specify quench and temper. You pay for a treatment you do not need, take on the distortion that comes with it, and then machine around a hardness you never wanted. If the part needs a hard, wear-resistant surface, do not specify spheroidize annealing on its own; that route softens the material, which is the opposite of what a wear surface needs. The pairing has to match the failure mode you are designing against, not the most impressive-sounding treatment on the list.

One caution on grade names: 4140, 1045, and 52100 are industry-standard examples a shop will recognize, not a grade table Gerdau publishes for this line. Confirm actual grade availability and diameter limits with the mill before you write any of them on the order.

Reading a Mill Cert When Suppliers Differ

Once a bar is quoted, how do you confirm on paper that the property you asked for is the property you got?

The certificate has to carry more than a number. It needs the hardness scale, HRC or HBW, because the two are not interchangeable; the test location, surface or half-radius; the specimen orientation and gage length behind any tensile figure; the impact test temperature; and the standard the tests were run to. A value with no scale and no location is not a specification, it is a starting point for an argument.

Supplier data sheets differ exactly here. One lists 28-32 HRC at the surface; another lists 260-300 HBW at half-radius. The numbers look like neighbors and on the printed page they almost overlap, but they describe different material. Hardness falls as you move from surface toward center in a quenched bar, and the two scales are not a one-to-one conversion. Roughly 30 HRC sits near 285 HBW, and that mapping shifts with section size. Before you compare two suppliers, convert to one scale and one location, or you are matching a surface reading against a core reading and calling it agreement.

There is one more line the certificate should carry: which treatment was actually run. Gerdau's six in-house processes mean the same bar can travel more than one route to reach a quoted hardness, so a document that says only heat treated leaves you unable to check whether the route you paid for is the route that was used. Ask for the process name, the austenitizing and tempering temperatures, and the quench medium.

The One Number to Pin Down Before You Order

Before the order is released, what single acceptance detail can you not leave ambiguous? The hardness or strength value and the location it is measured at, taken together, because either one alone is meaningless.

Here is why it is that item and not, say, the impact temperature. A shaft accepted on surface hardness alone can pass inspection and still have an under-treated core. Put it under alternating torsional load and the crack starts at the core, where the material is soft, and grows outward; the surface you measured stays clean and gives no measurable warning until the part fails. Gerdau's own wording is that heat treatment can alter hardness and tensile strength up or down, and the point is that the value is only as good as the location where it was taken. Confirm the scale and the location first, and everything else on the certificate becomes checkable.

So the question to put to the supplier is not whether the bar is heat treated, but on which hardness scale and at what depth the figure was taken. When that answer is written on the certificate, the impact temperature and the batch traceability are just cleanup. Until you have it, you are ordering a treatment name and hoping the part's center agrees with its skin.