Three primary criteria for selecting bar steels are 1) suitability for end use, 2) suitability for manufacturing process, 3) economical delivery of the requirements.
Suitability for end use includes appropriate mechanical properties, physical properties and chemical compatibility. Mechanical properties can include hardness, tensile and yield strength, ductility as measured by % elongation or % reduction in area, and / or impact properties. Mechanical properties can be achieved by chemical composition, cold work, or heat treatment. Note: properties need to match the environmental conditions of the intended end use… Physical properties that are often considered include magnetic properties for solenoid, actuator, or electronic applications. Process path of steelmaking can play an important role in determining these properties.
Suitability for manufacturing requires at least a cursory understanding of the intended process path. Will there be extensive stock removal by machining? Welding, brazing or other means of bonding? Heat treatment? Will the equipment used to machine require tight dimensional tolerances or straightness? Will the material be upset or cold worked? Will the material be cold worked (crimped, swaged, planished or staked) after machining? Bismuth additives can prevent achievement of bond strength in brazed joints unless special techniques and materials are employed. Various chemical constituents can have an effect on the cold work response of steel. These too can be determined by the melting and thermomechinical history of the steel before it arrives at your shop.
Economical delivery of requirements means choosing a materal that permits the creation of conforming parts that fully meet the requirements for end use and manufacturability at a total lowest cost. There are many ways to meet any particular set of requirements for steel in most uses. Chemistry, cold work, heat treatment, as well as design details can all be criteria used to select one material over another. Minimizing costs is clearly important, but most important is assuring that all of the “must have” properties (strength, hardness, surface finish, typically) needed in the finished product are delivered.
Costs of manufacturing can make up a large fraction of the final products cost. For some parts, the cost of manufacturing and processing can exceed the cost of the material. Choosing the lowest cost process path that will assure required properties often requires steel materials that are priced above the cheapest available. This is because free machining additives, or cold finishing processes can reduce cost to obtain desired properties or product attributes when compared to those needed to get hot rolled product up to the desired levels of performance.
Bottom line: Buyers may want to get the cheapest price per pound of steel purchased; Savvy buyers want to buy the steel that results in the lowest cost per finished part- assuring that costs are minimized for the total cost of production of their product. Understanding the role of steel making and finishing processes can help the buyer optimize their material selection process.
Photo courtesy of PMPA Member Corey Steel.
Tag: % Reduction in Area
“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.
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
