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

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.

© 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 ofsurface heat treatment or case hardening:

� Carburising

� Nitriding

� Cyaniding and carbonitriding

� Flame hardening

� Induction hardening

requires a change in composition

does not require a change in composition


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 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.


� 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.

Carburising Atmosphere/Media


Carburising

Structure of 0.2% carbon steel pack carburised at 925 C for 6 h and furnace cooled

Structure after Carburisation


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 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.

CASE(martensite)

CORE(ferrite and pearlite)


Carburising

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.

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.


Nitriding

Microstructure of a nitrided case


Two distinct zones in the nitrided case

� 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

original nitriding temperature) uses


Nitriding

Limitations over Carburising

� 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


Nitriding

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


� 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% Na2CO3, and 30% NaCl. Typical chemical reactions to occur:

2NaCN + O2 = 2NaCNO3NaCNO = NaCN + Na2CO3 + C + 2N

Cyaniding


� 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).

Carbonitriding



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.


Flame hardening

Typical work coils used for high frequency induction hardening and heat patterns developed by each unit


Induction and Flame Hardening

Next Class

MME 291: Lecture 17

Fracture of Metals

Documents

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