Question

a- Do you consider hardness an important mechanical property? why? (specify few examples).

b- Comprare and distinguish between the Vickers, Brinell, and Rockwell tets.

c- which of the following have the highest hardness and why?

Brass, Mild steel, Aluminum

d-What is the purpose of microscope used in Brinell hardness test?

Answer 1

a) Hardness is genrally very difficult to define but general defination of hardness is resistance of surface of material to scratch, abrasion as well as indentation. So it is a surface property. It is very important property for desinging structures, fatigue components etc. For examples cast iron nail can scratch on the stainless steel component, from this we can conclude that cast iron nail is more hard than stainless steel. One another example for importance of hardness in machining processes like turning, drilling etc. In those machining processes hardness of cutting tool shuold be more than the material to be cut, otherwise machining can't be done.

b) In vickers hardness test square based diamond pyramid indentor with included angle of 136 degrees between opposite faces. The load range is usually 1 to 120 kg.

HV- vickers pyramid hardness number

L- Applied load

d- diagonal length of squre impression, mm.

In Brinell hardness test hand operated vertical hydraulic press force a ball inro a specimen. For ferros metal ball diameter is 10mm with 3000kg load and 500kg for nonferros metals. For ferros metal ball is pressed for 10sec and for non-ferros metal 30sec.

L- Test load

D- Diameter of ball, mm

d- diameter of impression, mm.

In Rockwell Hardness Test uses different types of indentors for different materials. This test is carried out by slowly raising test specimen against the indentor until a fixed minor load is applied. this is shown on dial gauge. then the major load applied until dial pointer comes to rest, then major load is removed and minor load is syill acting, then rockwell hardness no is shown on dial gauge. ex. 60kg Major load with diamond cone indenter is used for extremely hard materials.

c) BHN for

1. Brass-192-202
2. Mild Steel- 144.55
3. Aluminium-99-101

From the above Tests Brass is the hardest material.

d) Microscope is ised in Brinell hardness test features a calibrated reticle in a flat feild optical system, which make accurate measurement of the indentation for brinell hardness testers. The reticle is calibrated in 0.1 mm units over a range of 6mm and the field of vision is 7mm. Measures the indentation in large scale for accurate measurement of BNH.