Feb_March_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 | F E B R U A R Y / M A R C H 2 0 2 0 4 3 12 A nnealing is a generic term denoting a treatment that consists of heating to and holding at a suitable tem- perature followed by cooling at an appropriate rate, primarily for the softening of metallic materials. Generally, in plain carbon steels, annealing produces a ferrite-pearlite microstructure. Steels may be annealed to facilitate cold working or machining, to improve mechanical or electrical properties, or to promote dimensional stability [1] . Parts are typically annealed either in batch furnaces such as bell fur- naces or continuous furnaces such as mesh-belt and roller hearth-furnaces in the presence of an atmosphere that is non-oxidizing or reducing to components. These atmo- spheres are either produced by on-site atmosphere genera- tors or suppliedby blending nitrogen andhydrogen. Because generated atmospheres are produced by partial oxidation of a hydrocarbon gas in air and the composition of both air and a hydrocarbon gas change continuously with time, it is difficult to produce and supply generated atmospheres with consistent quality and composition. This is the prime rea- son that most of manufacturers have chosen blended nitro- gen(N 2 )/hydrogen(H 2 ) atmospheres for annealing furnaces [2] . Depending on the type of material, different compositions of N 2 and H 2 must be used to achieve the desired, in most cases bright, sur- face finish. Although most people in the indus- try know how important the atmosphere is, in nearly all annealing processes the atmosphere itself is not closed loop controlled and is just introduced with a fixed blend. This means the introduced gas composition is set-up either for the worst-case scenario or, if any unexpected external influences like air ingress or re- sidual oil contaminates the atmosphere, oxidized and/or decarburized material will be produced leading to rejects or re- work [3] . This article describes how to effec- tively design and utilize a N 2 /H 2 annealing atmosphere. Based on years of field expe- rience with N 2 /H 2 annealing atmosphere, Air Products has developed an innovative closed loop atmosphere control system, monitoring and regulating the reducing potential of the N 2 based atmosphere for annealing carbon steel. OXIDATION AND DECARBUR- IZATION IN ANNEALING Annealing is a relatively simple heat treatment to perform, but there are still several factors that must be carefully con- UNDERSTANDING AND OPTIMIZING NITROGEN-HYDROGEN FURNACE ATMOSPHERES FOR ANNEALING Proper specification, measurement, and control of annealing furnace atmospheres are critical to reduce sooting, oxidation, sticking, and distortion. Liang He*, Thomas Philips*, Donald J. Bowe, Guido Plicht, and Mark K. Weise* Air Products and Chemicals Inc., Allentown, Pennsylvania *Member of ASM International Fig. 1 — Ellingham diagram [4] . 11