Tempering using polymers and salts

The Temprasud Group also guarantees the possibility of carrying out treatments of parts that require vertical positioning, and/or a different severity of the quenching medium, thanks to the use of quenching pits and tanks, with tempering mediums other than oil such as polymer and molten salts.

Capabilities

The department is located in the Unitrat (Bari)establishment. Below is the list of equipment present.

1 CIEFFE pit furnace model FVF for the execution of the austenitization, carburization and carbo-nitriding phases in a controlled atmosphere, characterized by:

  • double atmosphere control by CO/CO2 analyzer and oxygen probe;
  • computerized cycle management, recording of the cycles carried out and traceability of the loads;
  • double temperature control;
  • maximum load capacity 2,500 kg and overall dimensions of Ø 1,000 x H910 x 2,100 mm;

3 pit furnaces Ø 1,000 x h 2,100 mm for the execution of the pre-heating and tempering phases with computerized cycle management, recording of the cycles carried out and traceability of the loads;

2 pit FURNACES for performing the austenitization phases in a protective atmosphere in manual flow adjustment with load capacity up to 2,000 kg.

1 washing machine for carrying out the pre-washing and post-treatment washing phases of the loads with immersion functionality.

1 Tank for carrying out the polymer tempering phase with the possibility of adjusting the polymer concentration, agitation and temperature.

1 Tank for carrying out the tempering phase in salt immersion with adjustment of agitation and temperature.

Treatments

The hardening and quenching heat treatment does not change the chemical composition of the starting material and is the combination of two phases: quenching and subsequent tempering.

The cooling speed must be all the higher the more the CCT (Continuous Cooling Transformation) curves are shifted to the left; therefore the critical quenching speeds are maximum for steels with basic hardenability and minimum for steels defined as self-hardening that allow a martensitic transformation even with cooling in calm air. The structure to be obtained is a tetragonal martensite.

The oil quenching treatment involves a complete austenitization at a temperature above Ac3, generally higher than 20° ÷ 50°C, followed by a rapid cooling in oil such as to exceed the critical quenching speed, so that the transformation takes place between Ms and Mf.

The martenistic salt quenching and tempering are a thermal treatment applied as interrupted quenching of steels typically in a bath of molten salt at a temperature higher than the martensite initiation temperature. The objective is to delay cooling for a certain period of time to render the temperature uniform throughout the piece. This reduces to a minimum the risk of deformation, fracture formation and residual stresses.

The martensitic quenching in polymer-in-water emulsion can be superficial or deep and the cooling rate is regulated according to the concentration of the polymers in water. The disadvantages of polymers concern the risk of fracture formation and increase in residual stresses due to the severity of the quenching.

The tempering is a complementary subsequent treatment to the hardening and is carried out in FVF furnaces. The tempering temperature depends on the final characteristics to be achieved. For that purpose, we distinguish:

  • Stress relieving tempering: to be performed at temperatures between 150 and 250°C. It aims at the transformation of tetragonal martensite (hard and brittle) into tempered martensite.
  • Softening tempering: it is carried out at moderately high temperatures (550 ÷ 650°C). It aims at the transformation of tetragonal martensite into sorbite (through the coalescence of finely dispersed cementite) with a reduction of hardness and obtaining an increased toughness.
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