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 4 12 13 sidered and controlled. In the furnace, the material under- goes a temperature gradient during the heating and cooling phases of the process. Due to external influences such as air ingress or oil vaporization in closed furnaces, there are also gradients of atmosphere composition. For example, in a continuous furnace the entry and exit areas are critical. In both areas oxygen (O 2 ) ingress can create an oxidizing at- mosphere. In the low temperature range a direct reaction of the metal with O 2 (reaction 1) can happen; in the hot zone, O 2 firstly reacts with H 2 to water (reaction 2), which impacts the oxidizing potential of reaction 3. In N 2 /H 2 atmospheres the oxidation by CO 2 normally can be neglected (reaction 4 and 5): Oxidation by oxygen: 1) 2 Me + O 2 → 2 MeO Oxidation by moisture: 2) 2 H 2 + O 2 → 2 H 2 O 3) Me + H 2 O → MeO + H 2 Oxidation by carbon dioxide: 4) 2 CO + O 2 → 2 CO 2 5) Me + CO 2 → MeO + CO Oxidation-/reducing potentials that need to be monitored/ controlled: for reaction 1: pO 2 for reaction 3 K H = pH 2 /pH 2 O for reaction 5: K C = pCO/pCO 2 The oxidizing and reducing potentials of the reactions 1, 3, and 5 are different for each element. The Ellingham dia- gram (Fig. 1) shows the standard free energies for oxide for- mation for several elements based on the oxidation-/reduc- ing potential of the reactions 1, 3, and 5. Using the Ellingham diagram, process engineers can design the atmosphere after the material and alloying elements are known. To investigate how oxidation and decarburization hap- pen on steel parts in an atmosphere containing oxygen and moisture, a group of experiments were designed and run in a laboratory box furnace [5] . In Fig. 2 and 3, the surface oxidation and decarburization of different steels in N 2 /O 2 and N 2 /H 2 O atmosphere are observed and studied. All the N 2 /O 2 experiments involve 30 min. holding time at the designed conditions. Gray oxidation surface (oxide scale) is produced on all samples. In heat treatment, the de- carburization process is sensitive to temperature [6] .The tem- perature can affect decarburization in three ways: 1) The dis- solution rate of cementite and the diffusivity in both phases (austenite and ferrite) increases with temperature, contrib- uting to a deeper decarburized layer after a given time. 2) The austenite fraction increases with temperature. Because carbon diffuses slower in austenite than in ferrite and ismore soluble in austenite, the presence of austenite reduces the thickness of the decarburized layer. 3) The reaction kinetics between oxidizing gases and carbon/iron is related to the temperature. (1) (2) (3) Fig. 2 — Surface microstructure of 1045 steel samples after being held in different N 2 /O 2 atmosphere for 30 minutes (1. N 2 -2000ppmO 2 at 500°C, 2. N 2 -2000ppmO 2 at 725°C, and 3. N 2 -1000ppmO 2 at 860°C).