Lec 15 surface hardening of steelsteacher.buet.ac.bd/bazlurrashid/291_2011_feb/Lec_15.pdf · MME 291: Lecture 15 Surface Hardening of Steels Today’s Topics ... at high temperature
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Prof. A.K.M.B. RashidDepartment of MMEBUET, Dhaka
MME 291: Lecture 15
Surface Hardening of Steels
Today’s Topics
� Surface hardening fundamental� Carburising� Nitriding� Cyaniding and carbonitriding� Induction and flame hardening
Reference:
1. S.H. Avner. Introduction to physical metallurgy, 2nd Ed., Ch. 8, pp. 315-336.
� After carburisation, the surface of the steel has about 0.8 to 1.2 % C and becomes hardenable (i.e., has the capacity of forming a fully martensitic structure), but not yet hardened.
� Hardening heat treatment is done after carburisationto transform the austenitic structure of the caseinto martensitic.
���� During carburisation, the composition of core remains the same (i.e., at about 0.2 % C). Thus, the core of the steel is not hardenable due to low C level.
���� During hardening, the austenitic structure of the core transforms into ferrite-pearlitic.
���� The nitriding temperature is usually about 500-550 C, which is significantly lower than the carburising temperature of 925 C.
���� The steel is not heated to the austenitic zone.
���� The main purpose of heating to high temperature is to increase the diffusion rate of N atoms.
���� No post-hardening treatment is necessary.
���� Best results are obtained during nitriding if one or more nitride-forming alloying elements (Al, Cr and Mo) are present in steel. Hardness up to RC70 can be obtained.
Nitriding Atmosphere/Media
� Mixture of ammonia gas and cracked ammonia
� Nitriding heating cycle can be very long, depending
on the case depth required.
A 60-h cycle produces a case depth of only about 0.024 inch at 500 C.
� A “ white layer ” containing nitride compounds of iron and other alloying elements. The thickness of this layer is 0.002 inch maximum. This layer is very brittle, and must be removed before using the article.
� Underlying the white layer, precipitates of alloy nitirdesonly are formed .
Nitriding
Advantages over Carburising
� Nitriding is performed at relatively low temperatures and
no quenching is required
���� minimum distortion
���� parts can be machined to close finish before nitriding
� Complex parts can be nitrided without difficulty
� Wear resistance is outstanding
� Hardness is unaffected during high temperature (below the
� Salt bath having low in C and high in H is used, compared to liquid carburising (where composition high in C and low in H is used).
� The heating temperature is about 760 - 875 °C, which is lower than that used in carburising.
� Exposure is for shorter time, resulting thinner case (up to 0.01 in for cyaniding, 0.03 in for carbonitriding).
� The case usually contains about 0.5-0.8 % C and 0.5 % N. Typical bath composition is: 30% NaCN, 40% Na2CO3, and 30% NaCl. Typical chemical reactions to occur:
� Also known as dry cyaniding, gas cyaniding, microcarburising, or ni-carbing, carbonitriding is a modification of gas carburising.
� Addition of anhydrous ammonia gas to the furnace atmosphere causes both C and N to be absorbed by the surface of steel at the carbonitriding temperature.
� Although a wide variety of gas mixtures are used, typical composition is: 15 % anhydrous ammonia, 5 % natural gas, 80 % carrier gas (a mixture of N2, H2 and CO).
� Heating temperature range is 650 – 885 °C, lower than those used for gas carburising.
� Case depth rarely exceeds 0.02 in (due to lower heating temperatures).
In induction hardening, the surface of the steel is heated quickly using high-frequency (10-500 kHz) induction current, and then quenched in water.
Only surface of steel is austenetised during heating, so that martensite is produced only at the surface. The temperature of core remained below the lower critical and no change has occurred.
In flame hardening, the surface of the steel is heated quickly using oxyacetylene torch, and then quenched in water. A structure similar to that obtained in induction hardening is obtained.