General question about hardening and tempering process

[tcwong]

To all,

I am in the process of evaluating the performance of an existing half hard tempered CRS bracket and I just realized that I don't have any information regarding the physical property of the material after the hardening process other than the hardness number. I know there is a relationship between BHN and UTS, but is there anything else? How about the yield strength? Would the Young's Modulus be affected in anyway?

Any help will be greatly appreciated.

Thomas Wong
Chicago

 

[ubrales]

TCWONG, yes there is a relationship between BHN (Brinnell Hardness Number) and UTS (Ultimate Tensile Strength) - generally, the higher the BHN, the higher the UTS. And the BHN is negatively correlated to the toughness. When you go for higher hardness, you are sacrificing toughness and the part becomes more brittle (lower Izod impact value, lower Charpy test results). The grain size after hardening and tempering is also important. A fine grain is better. These characteristics are obtained by the alloying elements of the steel. You can get a lot more specific information from the steel manufacturer.

 

[borjame]

A number of material properties will be affected. The hardening procedure will affect YOung's modulus, yield strength, ultimate strength, etc. You need to find data for the material in the hardened state.

 

[TVP]

Thomas,

Be careful when using the word temper, since there are two meanings. One meaning is the heating process performed after austenitizing & quenching a hardenable steel alloy, so that the martensite is tempered to a more desirable microstructure. The other meaning is used to describe the condition of metals, for example, cold rolled-steel in the half hard temper or an aluminum alloy that has been processed to the T6 temper.

I believe it is this second meaning in which you are interested. If a metal sheet/strip is cold-rolled without subsequent annealing, it will be stronger/harder, but have less elongation and formability. Half-hard is not very precise, but it means that the steel has been cold-rolled so that the thickness has been reduced by a certain percentage. For low carbon steel, this means the hardness will be ~ 70-85 HRB, the approximate tensile strength is 380-520 MPa, and the elongation will be ~ 4-16%. Data is from ASM HANDBOOK Volume 1.

 

[tcwong]

TVP,

That's about the information I have come across so far by browsing thru websites, and I think your message has clarified that for me. So, by refering a part half-hard really only mean the strip steel (before forming) has been rolled so that the thinkness has been reduced by a certain percentage to gain hardness and tensile strength and with no heat treating process involved? I guess I got the two mixed up, I always thought that the terms full-half, half-hard and quarter-half came from heat treatment processes.

So, this half-hard bracket I am studying has a higher UTS, higher hardness number but with no improvement in Young's Modulus or yield strength? I am not sure if I understand what it means when you say "elongation will be ~4-16%"

Thanks,


Thomas Wong
Chicago

 

[TVP]

The elastic modulus (Young's modulus) is essentially invariate for steel with respect to heat treatment, rolling, etc. However, while the elastic modulus does not change, the yield strength most certainly changes-- it is more heavily affected by processing (heat treating, rolling, etc.) than the tensile strength. More cold rolling means a higher yield strength.

Elongation refers to the change in length of a sample that has been tensile tested to failure. The steel will stretch ~ 4-16% depending on the grade, amount of cold reduction, etc. The yield strength will be the stress required to cause permanent deformation and the ultimate tensile strength is the highest stress produced during the test. Refer to a text book or a perform a keyword search on Google for more information on tensile testing.