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材料的硬度

Hardness 硬度

Hardness is a measure of a material’s resistance to plastic deformation within a tiny zone of material surface where an indenter is driven into by a force. 

There are three hardness testing methods often used in practice according to the shape and size of indenters(压头):

  • Brinell hardness布氏硬度 

  • Rockwell hardness洛氏硬度 

  • Vickers hardness维氏硬度 


Note that, measured hardness values are only relative and take care to compare values determined by different techniques!

(1) Brinell Hardness (HB,布氏硬度)

 

In the Brinell hardness test, a ball made of hardened steel (淬火钢球)or tungsten carbide (硬质合金球) is pressed into the surface of a sample first, then unloaded after holding force for some time. An impress would leave on the surface where the ball was pressed into. 


Brinell hardness number (HB) is defined as that the force is divided by the spherical area of the indentation. The large this number, the harder the material.



      

                                           

HBW: a  tungsten carbide ball indenter is used for a materials below 650.

HBS: a hardened steel ball indenter is used for a materials below 450. 

The number before HBW or HBS means hardness, 

and the numbers after HBW or HBS mean diameter of the ball, load and holding tome of the load.

For example, 120HBS10/1000/30

It means hardness is 120 when the diameter of hardened steel ball is 10 mm, and a load of 1000 kgf (9800N) is kept for 30s. 

Brinell Hardness (HB)

Merits: small measuring error, stable data

Demerits: large indentation, unsuitable for thin parts, finish products and materials harder than the ball indenter.

(2) Rockwell Hardness (HRX,洛氏硬度)




In the Rockwell hardness test, the indenter is a standard diamond cone (金刚石圆锥) or hardened steel ball. The hardness is determined by measuring the depth of penetration of an indenter under a large load compared to the penetration made by a preload.







 


The Rockwell hardness (HR) is given by (k-h), where k is a constant determined by the type of indenter used and h is the depth of penetration.

HR=k-(h3-h1)/0.002



                     HR=k-(h1-h0)/0.002

The Rockwell hardness is classified into 9 scales according to the different loads or indenters (see P9 Tab.1-1), among which A, B, C are most usually used.

HRA, high hardness materials such as hard alloys, surface quenched layers and carburized layers.

HRB, low hardness materials such as nonferrous metals, annealed steels and normalized steels.

HRC, medium hardness materials such as hardened and tempered steels, quenched steels.

Rockwell Hardness (HRX)


Merits: easy to operate, small indentation, wide scope of applications.

Demerits: large dispersion degree of measuring results.

(3)Vickers Hardness (HVX,维氏硬度)

In the Vickers hardness test, the indenter used is a specially shaped diamond (顶部两相对面呈136º的正四棱锥金刚石压头) and again the area of the indentation is measured. The Vickers hardness number (HV) is obtained from the area and the force. 

Vickers is referred to as microhardness-testing methods on the basis of load and indenter size. It is well suited for measuring the hardness of small, selected specimen regions.




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