Vickers

It is generally used with half mild or mild steels (carbon <. 4). Its purpose is to enrich in carbon the superficial metal layers by diffusion phenomenon. To obtain a hard cemented layer after this processing, we generally proceed by tempering. The chemical processing increases the rate of atomic defects by the introduction of one or many elements in the superficial layers. We can reach surface hardnesses of about 800 VICKERS. 

Hardness, Mohs scale Hardness, Rockwell Hardness, Vickers ... 

Vickers hardness test Vickers hardness tests Vicor Vicryl... 

Siemens AG has been involved in R D on PFFCs, and Vickers Shipbuilding Engineering Ltd. (United Kingdom) is evaluating PFFCs from Ballard Power Systems for power generation. A 35-ceU stack was successfully tested for more than 300 h. Plans are under way to test a 20-kW PEFC. 

Many types of hardness tests have been devised. The most common in use are the static indentation tests, eg, Brinell, Rockwell, and Vickers. Dynamic hardness tests involve the elastic response or rebound of a dropped indenter, eg, Scleroscope (Table 1). The approximate relationships among the various hardness tests are given in Table 2. 

Rockwell Hardness and Rockwell Superficial Hardness of Metallic Materials Test Method for Vickers Hardness of Metallic Materials Test Method Microhardness of Materials... 

Test Method for Vickers Hardness of MetaUic Materials... 

Vickers Hardness. The Vickers or diamond pyramid hardness (DPH) developed in 1924 was an improvement over the Brinell test. The Vickers test used a pyramidal diamond as the indenter. This permitted the hardness testing of much harder materials, and the constant 136° angle of the indenter eliminated the problem of variable indentation shape encountered using spherical indenters (1). 

Vickers hardness numbers are calculated from measurement of the indentation diagonals as follows, where HV = Vickers hardness,... 

The Vickers hardness test is a macrohardness test in which loads are commonly varied from 9.8 to 1180 N (1 to 120 kgf). Vickers hardness numbers are invariant with load within the stated limits. 

The Vickers hardness test is commonly made on a flat specimen on which the indenter is hydrauhcaHy loaded. When the desired number of indentations have been made, the specimen is removed and both diagonals of the indentations, measured using a caUbrated microscope, are then averaged. The Vickers hardness number may be calculated, or for standard loads taken from a precalculated table of indentation size vs VHN. The preferred procedures are described in ASTM E92 (2). 

The Vickers hardness test, developed in the United Kingdom, is more popular there than in the United States. VHN (Vickers hardness number) and DPH (diamond pyramid hardness) are synonymous terms. 

Surface finish requirements for the Vickers test vary with the test load. Heavy load tests can be made on a 120 grit ground surface. At low loads increasingly finer surface preparation is required, approaching that for metaHographic specimens, to permit accurate diamond indentation measurements. 

Minimum thickness requirements are 11/2 times the indentation diagonal measurement, and there should be no visible marking or bulge visible on the side opposite the indentation.The Vickers test is based on a plane surface however, correction tables are available for both convex and concave surfaces (see ASTM E92) (2). 

Conversion to other hardness scales from Vickers is approximated for specific materials Hsted in ASTM E140 (2). Conversions outside the stated areas should be avoided unless supported by test data. 

In time most commercially available microhardness testers accepted both Vickers and Knoop indenters. The Vickers remained almost universally used in Europe but shared acceptance with the Knoop in the United States. 

Although Vickers and DPH microhardness tests should yield the same numerical results on a given material, such is not always the case. Much of the observed variance may be a function of differences ia the volume of sample material displaced by the macro and micro iadentations. 

Fig. 3. Flame hardening (a) tempeiatuie—time heating curves of a 25 x 50 x 100 mm specimen at a feed of 75 mm /min and burner distance of 8 mm showing temperatures of A, surface B, 2 mm below surface and C, 10 mm below surface (b) hardness—depth curves for a 0.50% C steel 25 X 75 X 100 mm specimen at a feed of 50 mm /min, temperatures ia °C measured 10 mm below the surface, and burner distances ia mm, respectively, of A, 530 and 50 B, 540 and 12 C, 545 and 10 D, 550 and 8 and E, 565 and 6. Flame heating followed by water spray quenching. HV = Vickers hardness.

Eig. 8. Illustration of the effect of nitriding on the wear resistance of a steel blasted with steel grit A, 300 HV steel B, 750 HV steel case hardened by carburizing and C, 1100 HV steel nitrided at 500°C for 60 h (17). HV = Vickers hardness. 

The surface may gain a very high (eg, 1000 Vickers) hardness from this process. Surface deformation also produces a desired high compressive residual stress. Figure 9 illustrates the improvement in fatigue properties of a carburized surface that has been peened (18). 

Hardness. The Knoop indentation hardness of vitreous sihca is in the range of 473—593 kg/mm and the diamond pyramidal (Vickers) hardness is in the range of 600—750 kg/mm (1 4). The Vickers hardness for fused quartz decreases with increasing temperature but suddenly decreases at approximately 70°C. In addition, a small positive discontinuity occurs at 570°C, which may result from a memory of quartz stmcture (165). A maximum at 570°C is attributed to the presence of small amounts of quartz microcrystals (166). Scanning electron microscopic (sem) examination of the indentation area indicates that deformation is mainly from material compaction. There is htfle evidence of shear flow (167). 

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