Ezgi UĞUR - 030060195 8th Week

Boronizing

Boronizing is one of the recent methods of surface hardening, which may applied to any ferrous material but is generally adopted for carbon steels and tool steels. Both pack and gaseous techniques can be applied for surface hardening. In the case of pack process, the components are packed in heat-resistant boxes with mixtures of granules or paste of boron carbide or other boron compounds with additions of activators and diluents at 900-1000 centigrade degree. Boron diffuses inwards and iron borides (FeB and Fe2B) layers are formed. On the surface, FeB phase forms, while in the interior, Fe2B phase is formed. FeB phase is more brittle and is not desirable. Higher temperatures, longer treatment times and higly alloy steels favour teh formation of FeB phase. The boride layers are very hard. They have hardness which is even greater than 1500VPN.

The heat threatment time required for a case depth of 0.15mm is 6 hours at about 900 centigrade degree. This layer has high wear resistance, and such components are used in tractor parts, drop forging dies and jig bushes.

In this process, threatment temperature is very high, and hence hardening of components before boronizing is not required. Only in the case of tool steels after boronizing, hardening and tempering are required to have the desired mechanical properties.

(T. V. Rajan, C. P. Sharma, A. Sharma, Heat Treatment: Principles and Techniques, pp.163-164)

Chromizing

Like boronizing process, chromizing is also used for surface hardening of both carbon and tool steels.

There are two types of chromizing: pack chromizing and gaseous chromizing. The components to be chromized

are packed with fine chromium powder and additives. A typical chromizing mixture consist of 60 percent chromium

or ferro chrome (with carbon content not exceeding 0.1%), 0.2 percent ammonium iodide, and 39 percent kaoin

powder. Diffusion of chromium carbide is formed on the surface of steel. The threatment time needed for achieving

a case depth of 0.02-0.04 mm is 12 hours at 900-1020 centigrade degree. Hardness of chromium carbide layer

is about 1500 VHN.

(T. V. Rajan, C. P. Sharma, A. Sharma, Heat Treatment: Principles and Techniques, pp.164)

Carbonitriding

Carbonitriding is one of the case hardening process in which carbon and nitrogen are diffused into

the surface of the component simultaneously at a predetermined temperature followed by

quenching.

The process is carried out in a controlled atmosphere so that both carbon and nitrogen are absorbed

simultaneously by teh heated component. The concentration of hardening elements is more at

the outer surface of the steel and decreases progressively towards the core. The carbonitrided

components may be subsequently heat treated so as to form a hard wear-resistant case of the

type normally obtained by treatment in a cyanide salt bath.

The process is used in the production of shallow cases on carbon and alloy steels. The treatment is

usually done at 850-900 centigrade degree using a carburizing gas with low additions of ammonia.

Ammonia dissociates into hydrogen and nitrogen, the latter reacting with the surface of the steel

to form nitrides. In addition, however, a small amount of nitrogen goes into solution increasing the

hardenability to some extent. The subsequent quenching ensures the final full skin hardness.

Carbonitriding results in a higher hardenability of the surface layer as compared to carburizing

and allows steel with lower content of alloying elements to be used.

The primary object of carbonitriding is to impart a hard case to the steel in order to provide

resistance to metallic and abrassive wear. The process competes with liquid cyaniding, carburizing

and to a lesser extent with nitriding.

( K. H. Prabhudev, Handbook of Heat Treatment of steels, p.386)

Chemical Blanking

Chemical blanking uses chemical erosion to cut very thin sheet -metal parts- down to 0.025 mm thick and/or for intricate cutting patterns. In both instances, conventional punch-and-die

methods do not work because the stamping forces damage the sheet material, or the tooling cost would

be prohibitive, or both. Chemical blanking produces parts that are burr free, an advantage over

conventional shearing operations.

Methods used for applying the maskant in chemical blanking are either the photoresist mothod or

the screen resist method. For small and /or intricate cutting patterns and close tolerances, the

protoresist method is used; otherwise, the screen resist method is used. The small size of the work

in chemical blanking exludes teh cut and peel maskant method.

Application of chemical blanking is generally limited to thin materials and/or intricate patterns.

Maximum stock thickness is around 0.75 mm. Also, hardened and brittle materials can be processed

by chemical blanking where mechanical methods would surely fracture the work.

Tolerances as close as +- 0.0025 mm can be held on 0.025 mm thick stock when the photoresist

method of masking is used. As stock thickness increases, more generous tolerances must be allowed.

Screen resist masking methods are not nearly so accurate as photoresist. Accordingly, whwn close

tolerabces on the part are required, the photoresist method should be used to perform the masking

step.

(M. P. Groover, Fundamentals of Modern Manufacturing: Materials, Processes, and

Systems, pp.638-640)