May/June_AMP_Digital

FEATURE A D V A N C E D M A T E R I A L S & P R O C E S S E S | M A Y / J U N E 2 0 2 0 6 1 passage and monitoring through the quench process whether water, salt or oil. The following case studies are examples of thru-pro- cess temperature monitoring surveys. CASE 1: LOW-PRESSURE CARBURIZING Carburizing has rapidly become one of themost critical heat treatment processes employed in the manufacture of automotive components. Also referred to as case hardening it provides necessary surface resistance to wear, while main- taining toughness and core strength essential for hardwork- ing automotive parts. The carburizing process is achieved by heat treating the product in a carbon rich environment, typically at a tempera- ture of 900 to 1050°C (1652 to 1922°F). The temperature and process time significantly influence the depth of carbon dif- fusion and associated surface characteristics. The next step is critical to the process. Following diffusion, the product is rapidly quenched and the temperature is rapidly decreased to generate the microstructure, giving the enhanced surface hardness while maintaining a soft and tough product core. An increasing trend in the carburizing market is the use of batch or semi-continuous batch low-pressure carburizing (LPC) furnaces. Following the diffusion, the product is trans- ferred to a high-pressure gas quench chamber where the product is rapidly gas cooled using typically N 2 or helium at up to 20 bars. 5 The technical challenge is twofold. The thermal barrier must be capable of protecting against heat during the carbu- rizing, and also against very rapid pressure and temperature changes inflicted by the gas quench. To protect the thermal barrier in the LPC process with gas quench, the barrier construction needs to be able to withstand constant temperature cycling and high gas pres- sures. The design and construction features include: • Metalwork – 310 stainless steel reduces distortion at high temperature combined with internal structural reinforcement. • Insulation – Ultra-high temperature microporous insulation minimizes shrinkage problems. • Rivets – Close pitched copper rivets reduce carbon pick up and maintain strength. • Lid expansion plate – Reduces distortion during rapid temperature changes. • Catches – Heavy duty catches eliminating thread seizure issues. • Heat sink – Internal heat sink to provide additional thermal protection to data logger. During the gas quench, the barrier needs to be protect- ed from nitrogen N 2 or heliumHe gas pressures up to 20 bar. Such pressures on the flat top of the barrier would create ex- cessive stress to the metalwork and internal insulation/log- ger. Therefore, to protect the barrier, a separate gas quench Fig. 1 — Typical thermal profile for thru-process temperature monitoring system through a continuous sealed gas carburizing furnace.