Why are these properties of steel important

Question

tensile strength, yield strength, ductility, and hardness

Answer 1

Why are these properties of steel important:

tensile strength, yield strength, ductility, and hardness?

These properties are important to the way steel is used in making things.  Steel can be specially mixed at the steel mill and then specially processed as products are manufactured from it in the factories that make the things we use to give the needed properties.

Steel is a mixture of iron and a small amount of carbon.  It can also contain other elements, especially metals to give it special improvements in properties. While the number of variations is almost endless the vast majority of the steel in our every day lives, in paper clips and nails; cars, trucks and appliances; buildings, bridges, railroads and ships has a form with around 0.2% carbon.  This common "carbon steel" is fairly ductile (as in a common paper clip easy to bend or reshape multiple times with out breaking).  It is low on the hardness scale scratching relatively easily and fairly useless for holding a cutting edge.  But it is stronger than most other common materials.

The properties of tensile and yield strength are important to the understanding of steel.  They are determined by metallurgists by stretching a round steel bar by pulling it apart in a special (and fairly large) machine called a tensile tester until in breaks.  The machine measures the pulling load all the way up to the breaking point.  The load at the breaking point is divided by the cross-sectional area of the bar to give tensile strength in pounds per square inch.

So what is yield strength?  Turns out that most materials have a load level (in pounds per square inch) below which they always return to their original shape.  Load them above that limit and their shape (in this case the length of the steel test bar) is permanently changed.  That limit is called yield strength.  This number is important to engineers that design things to be made from steel because they generally don't want to see their designs fail to stay in the right shape.

Carbon steel of the 0.2% variety has a yield strength in the range of 40,000 pounds per square inch (psi) and a tensile strength about 50% greater.  When you are shaping a piece of steel to give it a different form, whether it's a paper clip or a heavy steel bar, in order to get that different form part of the metal undergoes a tensile load above the yield strength.  The ductility of the material is related to the difference between the yield and tensile strength.  More ductile materials have a wider spread between yield and tensile strength.  Thus they can be formed without breaking.  Brittle materials with yield strength and tensile strength almost the same are difficult to form without breaking.  (Glass is an example).

Now how is steel made stronger?  The basic way starts with raising the carbon content in the original manufacture at the steel mill.  But there is more to it than that.  To get the high strength you not only need the extra carbon, you need to rapidly cool the steel from a high temperature to near room temperature.  This is known as heat treatment.  Steel with at least 0.4% carbon is able to develop full hardness with this treatment.  But following this step the steel is quite brittle and easy to break.  To regain some ductility for practical us a further heat treatment known as tempering is done.   The steel part is heated and held at a temperature of some 400 degrees F to 1200 F for several hours.  Lower tempering temperature retains the best tensile strength and hardness albeit with some brittleness.  The higher the temperature the lower the hardness but the better the ductility.

One other important consideration is that the harder the steel is after heat treating the more difficult it will be to shape especially with cutting, machining and forming processes.  Typically most shaping is done on a part before heat treating and if any final precision machining is needed due to distortion in heat treating it is usually done by grinding.  Generally chemical based fabrication processes like plating or etching or modern high energy processes like laser cutting are unaffected by steel hardness but may produce unacceptable changes in the hardened steel.

Another point is that a wide variety of improvements can be made by adding alloying elements to steel as well as advanced and specialized thermal treatments.  In general there is a large body of information available on properties, specifications and applications of the AISI alloys as well as proprietary alloys.

Ed Weldon

My apologies to the technical types who read this and are dismayed at my use of English measurements instead of the Metric units that are a more common part of their professional life.  The intended audience for my comments are a generally less technically sophisticated community.