November/December 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 | N O V E M B E R / D E C E M B E R 2 0 1 8 6 0 T his article explores the five common reasons why dis- tortion and cracking occur during atmosphere heat treatment and oil quenching in integral quench batch furnaces. It also shares best practices for addressing these is- sues and refining the carburizing and quenching processes. As a result, practitioners will be able to minimize part distor- tion during atmosphere heat treatment. Ultimately, the goal is to reduce or eliminate the associated post-treatment steps and manufacturing costs associated with correcting distor- tion. Two common, and often essential, processes per- formed in atmosphere heat treating systems are carburiz- ing and quenching. These processes have similar goals: to carburize parts uniformly and to produce a uniform quench withminimal part distortion and even hardness distribution. REASONS FOR DISTORTION DURING HEAT TREATMENT Distortion is a term for all types of irreversible dimen- sional changes that occur in a work piece. Two common types of distortion that occur during the heat treatment pro- cess are size distortion and shape distortion. Total size dis- tortion is the sum of distortion arising during the heat treat- ment cycle. Shape distortion is created by nonuniformity of temperature during phase transformation. Total size dis- tortion is equal to the sum of the distortions that take place during heating and cooling. Changes in dimensions are due to structural transformation and are characterized by mate- rial shrinkage or expansion. There are numerous reasons for distortion, ranging from material machining to heat treatment. This article fo- cuses on five common reasons for distortion and cracking during heat treatment: 1. Nonuniform and uncontrollable heat transfer during heating and cooling. 2. Structural changes during phase transformation. 3. Residual stresses in the part that occur due to forming, welding, or machining before heat treatment. 4. Macro or micro steel structure. 5. Fixturing and loading errors. DISTORTION DUE TO NONUNIFORM HEAT TRANSFER The maximum residual stress attained during quench- ing increases as the quenching temperature and quenching power of the coolant are increased. The nonuniform heat transfer during heating and cooling can sometimes be at- tributed to poor furnace design. According to AMS 2759/7B, the carburizing temperature should be consistent with the FIVE STRATEGIES FOR STREAMLINING ATMOSPHERE HEAT TREATMENT Learn about best practices for minimizing distortion and reducing manufacturing costs. Janusz Kowalewski Ipsen USA, Cherry Valley, Illinois 8 Fig. 1 — Gear manufacturing process.