Jeev4 --

The factors you mention tensile strength, etc., are relatively unimportant in the selection of steel for most applications. So for the moment just ignore the charts that show properties and all the alloy types. As your engineering and design work becomes more sophisticated you can go back to those charts.

Generally our choice of a particular alloy of steel is based on cost, availability and how we will use it in fabrication. The form of the steel and how it is processed at the steel mill is important to how we will process it in manufacturing and to a smaller extent how it will perform in service. These considerations are what will govern your steel specification.

And note that steel technology has been with us for about 160 years. The evolution of it's use in engineering is long and should be respected. In other words if a particular type and form of steel has come into common use for some specific application then this is worthy of serious respect from the engineer. You are best advised to thoroughly understand what you are doing when contemplating a change.

Also in design you should respect traditional factors of safety used in design of steel members. Much is covered by codes. But in some applications such as smaller machinery good design practice or a liberal factor of safety is called for.

As a matter of machine design practice I've always kept design stresses under 10,000 psi for low carbon steel members with a yield strength of 40,000 psi. Anytime I would need to go higher in stress would be a signal to analyze the design more carefully to understand combined stresses, fatigue, stress risers, corrosion and anything else that might affect the design. So usually with a good safety factor the issues of design in steel become more related to deflections and other design issues rather than stress.

If you find yourself designing for hardened steel applications, especially in machinery you are advised to become well versed in areas of machine design related to fatigue strength, stress risers, etc. Time spent in study of the teachings of Spotts or Shigley or their modern contemporaries is advised.

Five different general shape forms of steel are in common use:

1. Sheet steel up to perhaps 5mm thick as cold rolled. Available mostly as cold rolled in sheets or coils and specials with coatings. Sheets of hot rolled 1.5mm to 6-7 mm are occasionally available.

2. Bars usually round, rectangular, square and hexagonal are straight and perhaps 6-7 meters long from the supplier. Both cold and hot rolled are available. Round bars are the most common form of harden able and low alloy steels.

3. Tubes are either round (most common including steel pipe), square or rectangular. The manufacturing method of the pipe either forming and welding or cold drawing over a mandrel has effect on quality and price. Almost all are low carbon steel although there is some availability of small tubes of low alloy steels.

4. Hot rolled plates range from about 4mm thick to upwards of 200 mm.

5. Wire is usually cold drawn and shipped in coils. Usually round; but secondary suppliers often can supply just about any cross section shape that dies can be made for if order quantities justify.

6. Structural steel comes in a variety of hot rolled standard shapes like angles, channels, I-beams and lots of others. It's all a low carbon steel made to a variety of specifications, (ASTM in the USA), for various applications. It generally requires mill scale to be removed to get reliable adhesion of coatings for rust protection.

You need to understand some things about steels:

1. The vast majority of the steel (in the forms described above) we use to make things is low carbon steel around 0.2 percent carbon and a few other elements that little to do with the properties the design engineer is concerned with. Low carbon steels are either hot rolled or cold rolled. Hot rolled sheet has a mill scale that usually has to be removed in manufacturing to get best performance from coatings. Cold rolled has a clean surface, better tolerances and costs more than. Low carbon steel is easy to cut, machine, form and weld and it's properties are essentially unchanged by these processes. This greatly simplifies the specification of this material for the engineer. And in most uses it will deform at failure rather than break. Low carbon steel cannot be heat treated for improved "strength" but it can be given a thin hard wear surface by rather expensive processes called "case hardening".

2. All low carbon and low alloy steels (but not stainless steels) have about the same relative stiffness up to the point where they permanently deform. We call that "modulus of elasticity". Of course the amount of bending under load depends on the dimensions of the steel part. But keep the dimensions the same and any low alloy steel regardless of the hardness will bend the same amount as long as it doesn't permanently deform (exceeds it's "yield strength")

3. The principal way you increase the hardness and tensile strength (the two are directly related) of steel is by heat treating it. More carbon than 0.2% enables the heat treater to get more strength and hardness. Too much hardness will make the steel brittle and sensitive to breakage from many different causes. None of this changes the modulus of elasticity. Microstructure of the steel changes through the heat treating process. Our ability to observe and classify microstructure enables us to understand the properties and behavior of heat treated steels. Generally the engineer avoids specifying microstructure in specifics.

4. A few of the uses for steel such as tools, machinery or structure components where the amount of steel used needs to be minimized for cost or performance and fasteners require some tradeoff between "tensile" strength (actually yield strength in tension) and toughness. Alloying elements like nickel, molybdenum, chromium, vanadium, manganese and a number of others are added to steel in small percentages to make custom alloys for various of these needs.

5. All low alloy steels rust relatively easily and in most uses require some coating unless they are protected inside machinery from exposure to moisture.

6. Stainless steels contain more than 10% chromium. Adding nickel to that in amounts over 6% improve the corrosion resistance and begin to confer some new mechanical properties that are largely the result of the different microstructure (austenitic) of such stainless steels. There are many stainless steel specialty alloys designed to provide specific chemical resistance in combination with desired increases in strength.

Hope this is helpful to you --

Ed Weldon