FIELD OF INVENTION

The invention in general relates to the manufacturing process of special steel alloy witli specific properties. More particularly, it relates to the metallurgical process involved in the preparation of 13-8 precipitation hardening stainless steel, which is having properties such as high strength, stress corrosion cracking resistance and adequate toughness finds use as ball screws.

BACKGROUND OF THE INVENTION

Martensitic Precipitation Hardening Stainless Steet in particular 13-8 PH has got excellent transverse mechanical properties combined with good ductility and toughness in large sections. It is highly resistant to general corrosion and to stress corrosion cracking. It has optimum general corrosion resistance when it is fully hardened. Corrosion resistance is greatest in H 950 condition and decreases marginally at higher ageing treatment. MDN138 has good fabricability and can be age hardened by single treatment.
Cold working prior to ageing enhances the strength. The steel tmds use in applications that require a combination of stress corrosion cracking resistance and high strength. However, certain applications require the material to possess adequate toughness, even though the same is not specified by any international specifications.

SUMMARY OF THE INVENTION

It is one aspect of the invention to obtain Precipitation Hardening Martensitic Stainless Steel alloy (MDN 138) with stress corrosion cracking resistance and desired strength.

Another aspect of the invention is to achieve the property of good toughness in both direction of the alloy by composition balance designed to prevent formation of delta ferrite in the structure, lower carbon content to minimize grain boundary carbide precipitation and double vacuum melting to reduce alloy segregation.

A further aspect of the invention is to improve the toughness by thermo-mechanical processing and introduction of an intermediate annealing cycle.

DETAILED DESCRIPTION OF THE INVENTION

The alloy is double vacuum melted, Vacuum Induction Melting followed by Vacuum Arc Remelting (VIM/VAR). It develops yield strength upto 1500 MPa and good ductility properties irrespective of grain orientation in large sections. Good toughness in transverse direction is achieved by composition balance designed to prevent formation of delta ferrite in the structure. The carbon content is lowered to minimize grain boundary carbide precipitation and double vacuum melting to reduce alloy segregation and non-metailic inclusion content.

Studies have shown that low carbon, phosphorus, sulphur, silicon, manganese, nitrogen and titanium have beneficial effect on toughness of the steel. By careful selection of raw materials and meticulous processing the toughness property of the steel is enhanced.

The Ml' (Martensitic finish) temperature is near room temperature and heat treatment consists of a solution treatment followed by ageing. Cooling rates from the solution annealing temperature are not critical and heavy sections are air-cooled. The strengthening mechanism during ageing is attributed to the precipitation of aluminium containing intermediate compound in tempered martensite matrix. This alloy is generally supplied in the solution treated Condition A, ready for fabrication and subsequent hardening by the user. However, the material is also supplied in solution treated and overaged conditions for cold heading or forging.

The material is fully austenite above 816 C, Ms is approximately 121 C but varies slightly with composition and martensite transformation is practically completed below 21 C. Acl and Ac3 is 638 C and 690 C respectively. Ageing precipitates intermediate compounds. Solution annealing is normally earned out at 927 C.

Refinement in structure can be obtained by thermal cycling at a temperature just above the Ac3 when the precipitates are in solution. Such a practice has been adapted to improve the micro structure and thereby mechanical properties.


* As per international specification

TERMINOLOGY/ CHEMICAL FORMULAE TNVOLVED;-

MPa --> Mega Pascals

CLAIMS:-

1) A method for manufacture of a precipitation hardening stainless steel alloy with improved strength & toughness.. The method comprises of the following steps,

- Achieving the ingots of the alloy by Vacuum Induction Melting followed by Vacuum Arc Remelting to impart good ductility.-

- Avoidance of formation of delta territe during the forming process of the alloy by
maintaining balance in the composition of the constituents of the alloy and controlled hot processing,

- Lowering of carbon content to minimize grain boundary carbide precipitation.

- Imparting adequate toughness to the alloy by introduction of an Intermediate Annealing Cycle.