Some modern alternatives to conventional vacuum heat treatment technology

Basically, heat treatment processes should be described according to four technical fields which are combined in order to build the appropriate installation:

1) heating,
2) regulating a given gas or vacuum environment,
3) cooling,
4) ensuring the mechanisation of the parts.

Furthermore, uphill and downhill constraints have to be taken into account : parts flow, parts handling, post processes, etc. to make sure all economical and production aspects have been considered.

Although vacuum technologies have proven to be reliable in many cases, we will just focus on three alternative processes which are conventionally performed under vacuum. With respect to vacuum, the first example results in improved mechanical properties, the second and third examples show that costs can be drastically reduced by increasing the production and limiting handling operations.

1. Die hardening

The hardening of hot work dies with high toughness (i.e. 1.2365, 1.2343, 1.2344) is very sensitive and following aspects should be considered: avoid carbides precipitation, avoid decarburation and carburation, control the heating and the quenching phases. The cooling (quenching) should respect following requirements:

• cooling rate : min. 40°C per second to avoid carbides precipitation,
• enabling perfect cooling down homogeneity (distortion, residual stresses),
• final structure should be martensitic with pro-eutectic carbides.

The best solutions to fulfil the cooling requirements and get the better mechanical properties is quenching/hardening in molten salts:

• very high cooling rate if requested by the CCT diagram,
• flexibility: temperatures from 180°C to 550°C, fluid speed,
• no variation of the heat transfer coefficient (vapour phase, etc.),
• avoid any bainite formation (dramatic drop of mechanical properties up to 20-25%),
• perfect cooling homogeneity through the overall part or load,
• reproducibility of the treatment parameters and results.

PROFITHERM LINE, the SOLO Swiss multitreatment furnaces enable the integration of bell type furnaces under neutral or controlled atmosphere, which co-operate with salt quench tanks. Such a system also permits the so called “double-quenching” process to optimize the mechanical properties. With respect to vacuum treatment, this new technique allows a better control of all treatment parameters and improved mechanical properties together with limited distorsion. Furthermore, the salt-washing in closed circuit with salt recuperation is available.

2. Cu-Stainless steel brazing at T>1130°C of large parts (i.e. heat exchangers)

Under vacuum, production level is affected by the mechanisation of the parts through the heat treatment processes (discontinuous). On the other hand, transport of heavy parts at high temperature with a belt is not interesting since the belt will deform and have short life time.

The SOLO Swiss solution for this problem is to use steering rolls mounted on a gas tight furnace under controlled atmosphere. This technology presents many advantages since the production rate is very high, there is no need to heat up and to cool down transport means (belt, trays), both electrical and gas heating are available.

Such a technique can be applied up to 1200°C under any type of controlled atmosphere. The roll-transport technology enables also to manage the parts movement through the channel at various local speeds according to the special metallurgical requirements.

As a result, the production costs are drastically reduced and the flexibility of the installation ensured.

3. Nitriding of steels and stainless steels with controlled layer

It is classically assumed that only plasma nitriding enables to control all process parameters in order to regulate the type and nature of the nitriding layer which is built. Recent scientific works have proven on the other hand that similar and reproducible results can be obtained with gas nitriding. It is then possible to control the type (alpha-nitride, lambda-nitride or epsilon-nitride) as well as their respective growth kinetics.

The SOLO Swiss nitriding installations are designed to respect all requirements imposed by the scientific research and the industrial production. The new Axron Swiss process control manages all calculations and process parameters.

Furthermore, SOLO Swiss has developed new processes for stainless steel nitriding.

As a result, all handling and placing precautions which are necessary using plasma nitriding can be avoided and parts may be treated even in bulk, so the process can be fully automated.

Compared to plasma nitriding, this new generation of nitriding plants presents a much higher flexibility and productivity, since the nitriding cells also enable nitrocarburing with post oxidation and can be associated with cooling cells.