23-05-2012, 04:02 PM
HEAT TREATMENT OF“MILD STEEL”
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INTRODUCTION:
• Mild steels are often heat treated; the nature of this treatment depends on the type of steel and the reason for the treatment. These treatments, which include annealing, hardening and stress relieving, restore desirable properties such as corrosion resistance and ductility to metal altered by prior fabrication operations or produce hard structures able to withstand high stresses or abrasion in service. Heat treatment is often performed in controlled atmospheres to prevent surface scaling, or less commonly carburization or decarburization.
• Annealing
The austenitic stainless steels cannot be hardened by thermal treatments (but they do harden rapidly by cold work). Annealing (often referred to as solution treatment) not only recrystallises the work hardened grains but also takes chromium carbides (precipitated at grain boundaries in sensitized steels) back into solution in the austenite. The treatment also homogenizes dendrites weld metal structures, and relieves all remnant stresses from cold working. Annealing temperatures usually are above 1040°C, although some types may be annealed at closely controlled temperatures as low as 1010°C when fine grain size is important. Time at temperature is often kept short to hold surface scaling to a minimum or to control grain growth, which can lead to "orange peel" in forming.
• Cooling and Quenching
Martensitic stainless steels have high harden ability because of their high alloy content. Air cooling from the austenitising temperature is usually adequate to produce full hardness, but oil quenching is sometimes used, particularly for larger sections. Parts should be tempered as soon as they have cooled to room temperature, particularly if oil quenching has been used, to avoid delayed cracking. Parts sometimes are frozen to approximately -75°C before tempering to transform retained austenite, particularly where dimensional stability is important, such as in gauge blocks made of grade 440C. Tempering at temperatures above 510°C should be followed by relatively rapid cooling to below 400°C to avoid "475°C" embrittlement.
• Cleaning
Before annealing or other heat treating operations are performed on austenitic stainless steels, the surface must be cleaned to remove oil, grease and other carbonaceous residues. Such residues lead to carburization during heat treating, which degrades corrosion resistance.
• Pusher Type Furnace
Application
Normalizing of Cold Extruded Auto Parts Components
Atmosphere
Endo Gas
Max. Temperature
1000 oC
Capacity
550 Kgs Gross/Batch
Need For Heat Treatment Of Stainless Steel..
Stainless steels of various kinds are used in thousands of applications. The following gives a flavour of the full range:
Domestic – cutlery, sinks, saucepans, washing machine drums, microwave oven liners, razor blades
Architectural/Civil Engineering – cladding, handrails, door and window fittings, street furniture, structural sections, reinforcement bar, lighting columns, lintels, masonry supports
Transport – exhaust systems, car trim/grilles, road tankers, ship containers, ships chemical tankers, refuse vehicles
Chemical/Pharmaceutical – pressure vessels, process piping.
Oil and Gas – platform accommodation, cable trays, subsea pipelines.
Medical – Surgical instruments, surgical implants, MRI scanners.
Food and Drink – Catering equipment, brewing, distilling, food processing.
Water – Water and sewage treatment, water tubing, hot water tanks.
General – springs, fasteners (bolts, nuts and washers), wire.
Heat treating is a core manufacturing competency and can best be defined as “the controlled application of time, temperature and atmosphere to produce a predictable change in the internal structure (i.e. microstructure) of a material.
So thus heat treatment of stainless steel is of prime importance, as its very commonly used and can be fatal if we don’t know its characteristics.
Methodology-
• A specimen made up of stainless steel is taken.
• Its placed in the furnace and heated for the required time and temperature needed.
• The specimen is later cooled and the tests are conducted.
• The values are noted and tabulated
• The same procedure is repeated for various temperatures and similar tests are carried out.
• A graph is plotted and its characteristics are determined.