“Why do the mechanical properties on different shipments of the same size and grade of steel vary so much? “
To answer this, lets look at grade 1018, a non-free machining grade that we may encounter in our shops.
We'll pull it until its two pieces!
A cold drawn 1018 steel bar 1″ diameter typically has a Tensile Strength (TS) of 64000 psi. Yield Strength (YS) of 54,000 psi; %Elongation in 2″ (%EL) of 15%; % Reduction of Area (%RA) of 40%. (According to Information Report SAE J 1397, Estimated Mechanical properties and Machinability of Steel Bars,) Note, these are estimated values, not minimums!
Your mileage (properties) may vary- here are three reasons why.
- The original melt and cast process can affect chemical makeup;
- The mechanical properties of cold drawn steel are affected by the amount of cold work;
- The final steps of straightening and polishing can relax the steel.
The original melt and cast process can affect chemical makeup. Basic Oxygen Furnace (BOF) steels are made from a high percentage of new metal, and so have lower levels of residual elements from scrap that could strengthen the material. Also BOF steels tend to run lower levels of nitrogen, which is a ferrite strengthener. So BOF Melt steels tend to be on the low side of mechanical properties like tensile and Yield, and a bit higher ductility (%RA and %Elongation in 2″).
The mechanical properties of cold drawn steel are affected by the amount of cold work. This can come about in two different ways: the first way is as the bar size ordered gets smaller, given a standard draft, the percentage of cold work increases. This increase in the percentage of cold work increases the mechanical properties of Tensile and Yield Strength and can decrease the ductility somewhat.
The second way can be when different vendors use a different “drafting practice” resulting in a different amount of cold work to make the same size. Typical draft may be to use hot roll sized 1/16th” over the final size for drawing. Another vendor may choose 3/32″ oversize, and in rare cases a company my use 1/8″ to assure exceeding, not just meeting, minimum Yield Strength.
The final steps of straightening and polishing can relax the steel. The amount of cold work done in straightening the bars can relax the steel because the force is applied transverse to the original drawing. So a supplier using a two roll straightener, all other things being equal, may produce bars with a different final set of properties than one using a train of planishing discs to get the bar commercially straight.
So what values could you expect to encounter in grade 1018 steel when looking at all of these effects?
We’ve seen 3/8″ 1018 with Tensile Strength (TS) in the high 80,000′s; Yield Strength (YS) in the high 70,000′s.%EL in 2″ as high as 26;%RA as high as 65.
And in 4″ rd 1018, TS as low as 58,000psi; YS of about 42,000 psi; %EL in 2″ of 12%; % RA of 35%.
The process path generally can explain the properties received. And why those mechanical properties that you receive are sometimes so far from what you expect.
Photo credit: A-Lab Dayton Ohio