TPT January 2014

Article

Steel Plant Specialities

surface quality is deteriorated due to pitting. Metallurgical transformation during austenitising and subsequent quenching may be non-uniform. Surface hardness and strength are also lowered due to layer of scaling. Fatigue strength of heat treated product is reduced. This is especially true in case of automobile leaf springs.

Preventing oxidation and decarburisation

Prevention of oxidation and decarburisation is not only better than cure, it is profitable too. There are several ways to address problems caused by the two harmful reactions. Decarburised surface removal by machining operations after heat treatment, copper plating of thickness upto 0.025mm prior to heat treatment or change of heating media to molten salt bath are some ideas. A number of protective atmospheres may be introduced like liquid hydrocarbon, dissociated ammonia, exothermic gas, nitrogen and endothermic gas. Fluidised bed furnaces and vacuum furnaces have also proven to reduce scaling. Switching over to grades which do not require heat treatment is possible in rare cases. However, most of the mentioned solutions pose a number of problems or practical difficulties. Availability of capital and human resource for using high-end furnaces is a major issue. Many small heat treatment shops cannot afford these solutions. Yet they are under mounting pressure to prevent oxidation and decarburisation. Use of protective anti-scale coating has proven to be a logical solution to the problem of scaling and decarburisation. Insights into use of protective coating and its characteristics Use of protective coating has been found beneficial and cost- effective. An anti-scale coating is applied on components or billets to be heated before charging them into the furnace. This anti-scale coating acts as a barrier between oxygen and metal. Care is taken to apply a uniform, impervious layer of coating on the component to be heated. Coating ensures prevention of scaling and decarburisation. For exceptionally long heat treatment cycles of 10 to 15 hours, the extent of scaling and decarburisation mechanism is substantially reduced.

Figure 1 : Ability to salvage the rejected components

Anti-scale coating also reduces decarburisation on billets and ingots during hot forging and hot rolling operations. Heat transfer from heating media to metal is not affected due to anti-scale coating. No reaction with steel surface, no release of toxic fumes during use or heat treatment or storage, non-hazardous and economical implementation are other required characteristics of the coating. Coated tools and components must be able to be heat treated in air using a box type or bogie hearth; electric, gas or oil fired furnace. Benefits of anti-scale coating: industrial case studies and success stories Table 1 shows the efficacy of the coating in an electric furnace. Coating eliminates need of salt bath or controlled atmosphere equipment in many cases. Considerable savings in capital investment and operating costs are enabled by use of anti-scale coating. Due to prevention of decarburisation, uniform surface hardness is achieved. Rejected components can be salvaged (figure 1). Large savings are possible when plates of expensive alloy steel can be re-heat treated by using the anti-scale coating. Figure 2 explains the benefits of using coating during hot forming and solution annealing of stainless steel pipe fittings. Due to prevention of oxidation even in an ordinary oil fired furnace, pickling time could be reduced by 75 per cent. Buffing can be eliminated or minimised in many cases. In the manufacturing process of shearing blades of expensive high carbon, high chromium grade steel, grinding allowance is substantially reduced when protective coating is used during heat treatment. Some other distinct case studies are listed below. 1. Prevention of quench cracks Forgings like knuckle joints and crank shafts when heat treated in furnaces of oxidising atmosphere are susceptible

Table 1 : Efficacy of protective coating

Type of furnace used

: Box type, electric

Test coupon dimensions

: 300mm x 100mm x 10mm

Grade of steel

: AISI – 1010

Heat treatment cycle

: 1000°C / 4 hrs / air cool

% scale loss when not coated : 5.52 % scale loss when coated : 0.70

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January 2014