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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 6 14 alloying elements and lowest carbon content, 3) 52100 steel has highest carbon content, and 4) 4140 steel has medium alloying level and medium carbon content among the four steel samples. In the wet-N 2 experiments, weight loss (wt%) and de- carburization layer thickness (µm) were measured and the results are shown in Fig. 4. By analyzing the measurement results in Fig.4 with the composition information in Table 1, it can be concluded that: 1) higher carbon steel has higher weight loss because more carbon is easily taken out by ox- idation reaction with water (52100 steel has about 1% of carbon and showed highest weight loss rate); and 2) higher alloying levels help prevent the evolution of decarburization because the alloy elements help to hold carbon atom in car- bides and slowdown carbon atomtransportation in the steel (8620 steel has highest alloying level and did not show any measurable decarburization after the experiment). Comparing the two groups of experiments shows the difference between oxidation and decarburization by oxygen and moisture. A.S. Reeves and W.W. Smeltzer [7] studied de- carburization of steel containing 0.8% of carbon in oxidizing atmospheres. They found that iron oxidation was approx- imately tenfold more rapid than carbon oxidation at high temperature. In these tests, once oxidizing gases reaches the part surface, it will react with all alloying elements in the steel, including Fe, C, and Mn. If the iron oxide layer forms very quickly, it becomes a resistance layer for oxidizing gases reacting with C in the steel. Because carbon diffusion in iron oxides is very slow and solubility of carbon in iron oxides is very low, the decarburization process will be almost stopped once there is a relatively thick iron oxide layer. In the exper- iments, unlike oxygen, moisture is a relatively “soft” oxidiz- ing component in the atmosphere, so there is no thick oxide scale produced in wet nitrogen experiment, which allows continuous decarburization. OPTIMIZATION OF ANNEALING ATMOSPHERE The standard set-up of gas supply to the annealing fur- nace is usually with a liquid nitrogen tank, a liquid hydrogen tank (or H 2 -Packs) and a standard N 2 /H 2 blender to create a fixed blend to be fed to the furnace. As discussed before, although the flow rate and composition of nitrogen-hydro- gen atmosphere introduced into the furnace is fixed, the true 15 Fig. 4 — Surface microstructure of different steel samples after being held at 850°C for 80 minutes wet N 2 atmosphere. Fig. 5 — Example of optimized N 2 /H 2 annealing atmosphere.