Surface Treatments t of Steels Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Today s Topics Surface treatments fundamental Cyaniding and carbonitriding Induction and flame hardening Reference: 1. S.H. Avner. Introduction to physical metallurgy, 2 nd Ed., Ch. 8, pp. 315-336. Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 02 1
Surface Treatments Fundamental Many industrial applications require steels with a hard, wear-resistant surface and a relatively soft, tough inside. Examples: gear, cam shaft, roller, etc. Heat treatments like annealing or hardening are not suitable for such applications. There are FIVE principal methods of surface heat treatment or case hardening: Cyaniding and carbonitriding Flame hardening Induction hardening requires a change in composition does not require a change in composition Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 03 Carbon is added on to the surface of low-carbon steels (0.2 % C or lower) at high temperatures to make the surface hardenable. This process is called carburisation. The carburising temperature is usually about 925 C, where the structure becomes fully austenitic and has the potential of absorbing the maximum carbon atoms. Carburisation is then followed by hardening treatment (followed by tempering, if needed) when The structure of high-carbon surface (case) becomes martensitic, thereby increasing hardness. The structure of low-carbon inside (core) becomes ferrite-pearlitic, and remains as ductile and tough. Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 04 2
Atmosphere/Media Solid or pack carburising a mixture of charcoal, coke, and barium carbonate (about 20 wt.%) Liquid carburising a bath of molten cyanide and alkaline earth salts Gas carburising carbon monoxide and/or hydrocarbon gases Each carburising media provides atomic carbon, which diffuses into the steel surface during carburisation. The rate of diffusion of C in austenite depends upon the heating temperature and the carbon-concentration gradient. Under known operating conditions, amount of carbon penetration (case depth) with heating time can be predicted. Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 05 Structure after Carburisation Structure of 0.2% carbon steel pack carburised at 925 C for 6 h and furnace cooled Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 06 3
Heat Treatment after Carburisation 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 carburisation to transform the austenitic structure of the case into martensitic. During carburisation, the composition of core remains the same (i.e., at about t02% 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. CASE (martensite) CORE (ferrite and pearlite) Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 07 Nitrogen is added on to the surface of steels at high temperatures, where it reacts with iron to form iron nitride compounds, which are very hard. The nitriding temperature is usually about 500-550 C, which is significantly lower than the carburising temperature of 925 C. Here 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 nitrideforming alloying elements (Al, Cr and Mo) are present in steel. Hardness up to RC70 can be obtained. Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 08 4
Atmosphere/Media Mixture of ammonia gas and cracked ammonia 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. The nitrided case has two distinct zones: 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 nitirdes only are formed. Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 09 Microstructure of a nitrided case Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 10 5
Advantages over 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 original nitriding temperature) uses Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 11 Limitations over Corrosion resistance of steels is reduced considerably by nitriding (if the white layer is removed) Long heating cycle Formation of the brittle white layer Necessity of using special alloying elements to obtain high hardness Cost of nitriding atmosphere Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 12 6
Cyaniding and Carbonitriding Cases that contain both carbon and nitrogen are produced by liquid salt baths (cyaniding) or by use of gas atmosphere (carbonitriding). N imparts inherent hardness by forming hard nitride compounds, and increased C content makes the surface of steel hardenable during quenching Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 13 Cyaniding and Carbonitriding Cyaniding Closely related to carburising. 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% Na 2 CO 3, and 30% NaCl. Typical chemical reactions to occur: 2NaCN + O 2 = 2NaCNO 3NaCNO = NaCN + Na 2 CO 3 + C + 2N Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 14 7
Cyaniding and Carbonitriding Carbonitriding 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 N 2, H 2 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). Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 15 Induction and Flame Hardening No change in chemical composition of steel. The steel should be capable of being hardened (carbon content in the range of 0.3 to 0.6 %). 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. Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 16 8
Induction and Flame Hardening Flame hardening Typical work coils used for high frequency induction hardening and heat patterns developed by each unit Rashid, DMME, BUET. 2009 MME 291, Lec 15: Surface treatments of steels P 17 Next Class Nondestructive Inspection of Metals 9