CASE STUDY OF HARDENING

TABLES OF CONTENTPAGE

1) INTRODUCTION OF HARDENING2) THEORY BEHIND HARDENING3) HOW DOES HARDENING TAKE PLACE4) PROPERTIES5) HARDENABLE STEELS6) HARDENING PROCESSES7) TYPES OF HARDNESS MEASUREMENT8) TYPES OF HARDNESS TEST BRINELL HARDNESS VICKERS HARDNESS ROCKWELL HARDNESS9) CONCLUSION 10) REFERENCE

INTRODUCTION

Hardening is heating and then cooling at such a rate of steel alloys so that a significant increase in hardness occurs. In most cases, hardening is performed in connection with a subsequent new heating, the tempering. If, after hardening at a high temperature, tempering is performed and then a relatively low hardness combined with high toughness is achieved, this process is called hardening and tempering.THE THEORY BEHIND HARDENING.

Steel has in an unhardened state a body centered cubic (BCC) structure, in which it can only dissolve very little carbon. After warming up over approx. 720 C austenite is originated that has a body face centered (BFC) crystal structure (and occupies a smaller volume). It can dissolve considerably more carbon, which occurs at hardening temperature. By cooling the material then rapidly enough, converting it from a cubic face centered crystal structure back into a cubic spatially centered structure, oversatured carbon remain and martensite is originated. Due to the presence of supersaturated carbon, the BCC (body centered cubic) lattice is stretched out to a tetragonal lattice. The martensite thus possesses high internal stresses and a larger volume than non-hardened steel at room temperature. As a consequence, the high internal stresses have a high hardness of the material. Upon tempering, a little carbon will be diffused from the tetragonal cube. Consequently stress and volume, but also hardness decrease and the toughness increases significantly.HOW DOES HARDENING TAKE PLACE.

The hardening process of workpieces can be divided into three technical steps. First, the workpiece is heated so that the output structure transforms into an austenitic structure. This austenitizing temperature depends on the material used (750 C 1210 C). Then the workpiece is maintained at this temperature so that alloying elements can be incorporated homogenously in this austenitic structure. The final step is quenching the workpiece at such a cooling rate that a so-called martensitic structure is originated. The heating and temperature maintenance should be performed in a protective atmosphere to protect the material from oxidation and decarburization. Cooling can be performed in different media. The most common are: water, saline water, hardening oils, polymers, salt bath, nitrogen or argon. After hardening steel has a relatively high brittleness and so it usually cannot be case-hardened in this condition. Therefore, directly after hardening it must be tempered at least once, however, more times is better. Tempering is an annealing treatment at lower temperatures, when the largest internal stresses that arose during the hardening, decrease. This decreases the hardness in most cases, but on the other hand, the toughness increases significantly.

Properties Higher wear-resistance Higher hardness Improved resistance to deflection Improved resistance to fraction/tearing Improved resistance to chipping Improved ductility

HARDENABLE STEELS Hardenable steels contain at least 0.3% carbon. Examples of steels that can be hardened are: spring band steel, cold work steel, high-grade steel, roller bearing steel, hot work steel and tool steel. A large number of high-alloy, stainless steels and alloys of cast iron can be hardened.

HARDENING PROCESSES The Heat & Surface Treatment and hardening process applied by Mamesta are hardening under protective gas and vacuum hardening. Hardening under protective gas is hardening of a workpiece in an inert gas atmosphere. This process serves to protect the surface of the component against oxidation as well as against decarbonizing and carbonizing. By an adjustable carbon potential of the protective gas atmosphere, decarbonizing and carbonizing can be undone. Vacuum hardening is the hardening of components in a vacuum atmosphere (in a controlled vacuum) in which temperatures up to 1300 C can be achieved. Purpose of this treatment is to prevent any oxidation or to avoid other reactions on the surface of the workpiece. The advantage of vacuum hardening is that metals remain white and a further mechanical treatment is usually unnecessary. Also isothermal hardening, a special hardening technique is applied by Heat & Surface Treatment and Mamesta.

THREE GENERAL TYPES OF HARDNESS MEASUREMENT.1) Scratch hardness The ability of material to scratch on one another Important to mineralogists, using Mohsscale 1= talc, 10 = diamond Not suited for metal annealed copper = 3, martensite = 72) Indentation hardness Major important engineering interest for metals. Different types : Brinell, Meyer, Vickers, Rockwell hardness tests. The ability of material to scratch on one another Important to mineralogists, using Mohsscale 1= talc, 10 = diamond Not suited for metal annealed copper = 3, martensite = 73) Rebound or dynamic hardness . The indentor is dropped onto the metal surface and the hardness is expressed as the energy of impact.

TYPES OF HARDNESS TESTHardness tests can be used for many engineering applications to achieve the basic requirement of mechanical property. For examples : surface treatments where surface hardness has been much improved. Powder metallurgy Fabricated parts: forgings, rolled plates, extrusions, machined parts.

BRINELL HARDNESS J.A. Brinell introduced the first standardised indentation-hardness test in 1900. The Brinell hardness test consists in indenting the metal surface with a 10-mm diameter steel ball at a load range of 500-3000 kg, depending of hardness of particular materials. The load is applied for a standard time (~30 s), and the diameter of the indentation is measured. giving an average value of two readings of the diameter of the indentation at right angle. The Brinell hardness number (BHN or HB ) is expressed as the load P divided by surface area of the indentation.

Where P is applied load, kg D is diameter of ball, mm d is diameter of indentation, mm t is depth of the impression, mmADVANTAGES AND DISADVANTAGES OF BRINELL HARDNESS TEST Large indentation averages out local heterogeneities of microstructure. Different loads are used to cover a wide rage of hardness of commercial metals. Brinell hardness test is less influenced by surface scratches and roughness than otherhardness tests. The test has limitations on small specimens or in critically stressed parts where indentationcould be a possible site of failure.

VICKERS HARDNESS Vickers hardness test uses a square-base diamond pyramid as the indenter with the included angle between opposite faces of the pyramid of 136o. The Vickers hardness number (VHN) is defined as the load dividedby the surface area of the indentation.

Where P is the applied load, kgL is the average length of diagonals, mm is the angle between opposite faces of diamond = 136o.

Vickers hardness test uses the loads ranging from 1-120 kgf, applied for between 10 and 15seconds. Provide a fairly wide acceptance for research work because it provides a continuous scale of hardness, for a given load. VHN = 5-1,500 can be obtained at the same load level (easy for comparison).

IMPRESSIONS MADE BY VICKERS HARDNESS A perfect square indentation (a) made with a perfect diamond pyramid indenter would be a square. The pincushion indentation (b) is the result of sinking in of the metal around the flat faces of the pyramid. This gives an overestimate of the diagonal length (observed in annealed metals). The barrel-shaped indentation (c) is found in cold-worked metals, resulting from ridging or piling up of the metal around the faces of the indenter. Produce a low value of contact area giving too high value.

ROCKWELL HARDNESS The most widely used hardness test in the US and generally accepted due to:1) Its speed2) Freedom from personal error.3) Ability to distinguish small hardness difference4) Small size of indentation.

The hardness is measured according to the depth of indentation, under a constant load.

PRINCIPAL OF THE ROCKWELL TEST Position the surface area to be measured close to the indenter. Applied the minor load and a zero reference position is established The major load is applied for a specified time period (dwell time) beyond zero The major load is released leaving the minor load applied.

The Rockwell number represents the difference in depth from the zero reference position as a result of the applied major load.

ROCKWELL HARDNESS INSTRUCTION Cleaned and well seated indenter and anvil. Surface which is clean and dry, smooth and free from oxide. Flat surface, which is perpendicular to the indenter. Cylindrical surface gives low readings, depending on the curvature. Thickness should be 10 times higher that the depth of the indenter. The spacing between the indentations should be 3 or 5 times thediameter of the indentation. Loading speed should be standardised.

REFERENCE1) Dieter, G.E., Mechanical metallurgy, 1988, SI metric edition, McGraw-Hill, ISBN 0-07-100406-8.2) Walkerm P.M.B., Materials science and technology dictionary, 1999, Chambers Harrap Publisher, ISBN 0 550 13249 x.BNJ 10602 MATERIAL SCIENCEPage 2