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 5 13 14 (4) (5) (6) Fig. 3 — Surface microstructure of different steel samples after being held at 850°C for 80 minutes wet N 2 atmosphere, dew point = -4°C. (4. 1045 steel, 5. 4140 steel, 6. 8620 steel and 7. 52100 steel). The experimental results also show that temperature plays a very important role in the oxidation and decarburi- zation process to dominate the reaction kinetics. 500 o C is not high enough to allow steel to continuously supply iron and carbon to the surface to be oxidized. So, in condition-1, the sample only showed very thin blistering at the surface but no decarburization after being exposed to the N 2 /O 2 atmo- sphere for a short period. As shown in Fig. 2, decarburization can be easily observed at 860 o C in condition-3. However, at 725 o Cunder condition-2, noobviousdecarburizationwas ob- served, although more (double) oxygen was introduced into the furnace. This can also be explained by the temperature. In N 2 /H 2 annealing atmospheres, there is no free oxy- gen in the high-temperature zones because H 2 converts it into water. In the N 2 /H 2 O experiment, four different steels were exposed to a wet nitrogen atmosphere at 850 o C. The dew point of wet N 2 atmosphere was monitored and con- trolled at -4 o C. As each sample has different alloy elements TABLE 1 — CHEMICAL COMPOSITION OF FOUR STEEL TEST SAMPLES 1045 4140 8620 52100 C 0.43~0.50 0.38~0.43 0.18~0.23 0.98~1.10 Mn 0.60~0.90 0.75~1.00 0.70~0.90 0.25~0.45 Si 0.10~0.35 0.15~0.30 0.15~0.35 0.15~0.30 Cr 0.80~1.10 0.40~0.60 1.30~1.60 Mo 0.15~0.25 0.15~0.25 Ni 0.40~0.70 Fe rest rest rest rest (7) (Table 1), the near-surface microstructure is very different al- though they met the same atmosphere. From the information in Table 1, it can be seen that: 1) 1045 steel has lowest alloying level, 2) 8620 steel has most