There are several factors to be considered in preparing workpieces for vacuum heat treating or brazing. Cleanliness of the workpieces and baskets or fixtures is very important. They must be free of oil, dirt, machining lubricants or other contaminants prior to being loaded into the furnace. Some lubricants contain sulphur compounds which can adversely affect the alloys being heat treated. Inadequate cleaning can also cause staining and discoloring of the end product or result in poor braze alloy flow. Contaminants with high vapour pressures will evaporate during heating causing loss of vacuum. The vapours may eventually condense on colder surfaces in the furnace only to re-vaporize to cause contamination problems in subsequent runs. BY JEFF PRITCHARD
Archives for June 2014
Case Study: The Benefits of High Pressure Gas Quenching in Dimensional Control
Highly distortion prone gearing (Fig. 1) was the subject of an investigation into the dimensional changes which result from utilizing either oil or high pressure gas quenching following a low pressure vacuum carburizing process. For comparative purposes, the gears in question were also atmosphere gas carburized and plug quenched, which is standard practice for these geometries. Full production loads (Fig. 2) were run using two (2) different carburizing methods (atmosphere, vacuum) in combination with free quenching in either oil at 75°C (165°F) or high pressure gas (nitrogen) at 11 bar.
Gears were taken from multiple locations throughout each load for analysis. Parts for metallurgical evaluation were selected from the center of each load. Multiple areas on each part were then analyzed for microstructure, case depth, and hardness (surface, profile, core). Dimensional checks (out of round, gear tooth profiles) were conducted on the gears before and after heat treatment. For brevity, only a portion of the complete test program is presented here (see Reference 4 for more detail).