MME 291: Lecture 15 Surface Hardening of Steels Prof. A.K.M.B. Rashid Department of MME BUET, Dhaka Today s Topics Surface hardening fundamental Carburising Nitriding 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, 2011 MME 291, Lec 15: Surface hardening of steels P 02
Surface Hardening 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: ❶ Carburising ❷ Nitriding ❸ Cyaniding and carbonitriding ❹ Flame hardening ❺ Induction hardening requires a change in composition does not require a change in composition Rashid, DMME, BUET, 2011 MME 291, Lec 15: Surface hardening of steels P 03 Carburising Carbon is added on to the surface of low-carbon steels (0.2 % C or lower) at high temperature to make the surface hardenable. This process is called carburisation. The carburisingtemperature is usually about 925 C, where the structure becomes fully austenitic and has the potential of absorbing the maximum carbon atoms. Carburisationis then followed by hardeningtreatment (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, 2011 MME 291, Lec 15: Surface hardening of steels P 04
Carburising Carburising Atmosphere/Media ❶ Solid or pack carburising a mixture of charcoal, coke, and barium carbonate (about 20 wt.%) ❷ Liquid carburising a bath of molten cyanide and alkaline earth salts ❸ Gas carburising carbon monoxide and/or hydrocarbon gases Each carburisingmedia 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, 2011 MME 291, Lec 15: Surface hardening of steels P 05 Carburising Structure after Carburisation Structure of 0.2% carbon steel pack carburised at 925 C for 6 h and furnace cooled Rashid, DMME, BUET, 2011 MME 291, Lec 15: Surface hardening of steels P 06
Carburising 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 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, 2011 MME 291, Lec 15: Surface hardening of steels P 07 Nitriding 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. 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. Rashid, DMME, BUET, 2011 MME 291, Lec 15: Surface hardening of steels P 08
Nitriding Nitriding Atmosphere/Media Mixture of ammonia gas and cracked ammonia Nitridingheating 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. Rashid, DMME, BUET, 2011 MME 291, Lec 15: Surface hardening of steels P 09 Nitriding Two distinct zones in the nitrided case ❶ 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. ❷ Underlying the white layer, precipitates of alloy nitirdes only are formed. Microstructure of a nitrided case Rashid, DMME, BUET, 2011 MME 291, Lec 15: Surface hardening of steels P 10
Nitriding Advantages over Carburising Nitridingis performed at relatively low temperaturesand 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, 2011 MME 291, Lec 15: Surface hardening of steels P 11 Limitations over Carburising Corrosion resistance of steels is reduced considerably by nitriding(if the white layer is removed) Long heating cycle Nitriding Formation of the brittle white layer Necessity of using special alloying elements to obtain high hardness Cost of nitriding atmosphere Rashid, DMME, BUET, 2011 MME 291, Lec 15: Surface hardening of steels P 12
Cyaniding and Carbonitriding Cases that contain both carbon and nitrogen are produced by liquid salt baths (cyaniding) or by use of gas atmosphere (carbonitriding). Nimparts inherent hardness by forming hard nitride compounds, and increasedccontent makes the surface of steel hardenable during quenching Rashid, DMME, BUET, 2011 MME 291, Lec 15: Surface hardening 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
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, 2011 MME 291, Lec 15: Surface hardening 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, 2011 MME 291, Lec 15: Surface hardening of steels P 16
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, 2011 MME 291, Lec 15: Surface hardening of steels P 17 Next Class MME 291: Lecture 17 Fracture of Metals