Where Does 355 MPa Yield Strength Come From in 45 Steel?

In 45 steel, 355 MPa yield strength is a heat-treatment result written on the drawing as an acceptance value. It does not come with the grade. The number holds when the 25–30 mm section is fed, the quench and temper window is kept, and a test bar cast with the part is pulled to ASTM A370.

How the 355 MPa Figure Enters the Drawing

GB/T 699-2015 settles which steel this is: the grade, the composition behind it, and the quenched and tempered condition it ships in. It does not set a yield for a bracket with a 25–30 mm load path. The steel answers one question, and the heat treatment answers the other.

On the drawing, 355 MPa sits beside hole pitch ±0.2 mm and mounting face flatness 0.1 mm. All three are design targets for this bracket, still to be confirmed with the foundry, which puts yield in the inspection column, where a batch is accepted or rejected against it. A drawing that reads "45 steel, quenched and tempered" and stops there carries no strength acceptance.

Why the Section, Not the Grade, Sets the Limit

The bracket runs 180×120×60 mm at 1.8–2.5 kg, with walls of 8–12 mm and a 25–30 mm section where the load path reaches the bolts. Quenching drops the skin fast and the core last. In that section the core can miss the transformation, and tempering cannot put back a yield the quench never delivered.

That is the first failure mode for this part: the thick section does not harden through, and body yield lands short of 355 MPa. The reflex fix makes it worse: adding wall to the boss slows the core further and buys weight instead of strength. A riser feeding that section, with the body read instead of a coupon cut elsewhere, is what moves the number.

Quench 840–860, Temper 550–600, HB 220–250

The working window is a quench from 840–860 °C, a temper at 550–600 °C, and hardness at HB 220–250. Read the three as one setting. Temper temperature trades hardness for toughness, and hardness is the only one of the three the shop can read on the part itself.

Push the temper above the window and hardness falls out of HB 220–250, taking yield with it. Pull it below and the bracket stays hard and brittle, and the interrupted cut on the mounting face starts chipping. A reading inside HB 220–250 does not prove 355 MPa on its own. It shows the body saw the same temper cycle as the bar.

What the Test Bar Proves

A cast-on test bar is poured with the bracket, stays with it through the furnace, and gets pulled to ASTM A370. That chain is evidence for one heat and one temper cycle. It says nothing about the core of the 25–30 mm section, because the bar is thinner than the section it stands for.

One recorded example of that gap: a bar that passed while the body came in under 355 MPa. The correction was a cast-on bar, a riser on the thick section, and a temper adjustment. A bar poured from another heat, or a coupon cut from the bracket, breaks the chain at its first link, since neither shared the furnace load.

What to Put in the Purchase Specification

The table below carries the strength lines with the values this part uses. The mounting face belongs in the same list, because this bracket gets machined and finished after it is cast. It is cut after blasting to Sa 2.5, since blasting takes metal off and any dimension taken before it is a guess.

Ambiguity in the surface callout is where batches stall. Carbon steel normally goes to phosphating or black oxide, so a drawing that says passivation with no note behind it comes back as a question, and the answer holds the release. Send the strength lines, the machined face data, and one word for the surface treatment to the metal casting service that quotes the part. Then a bar that pulls under target is a rejection instead of a conversation.