Cr23C6 carbides in 1Cr22Mn16N steel


Figure 1: Microstructure of high nitrogen steel. Scale bar: 50 µm.


Figure 2: Schematic illustration of simulated welding thermal cycling.


Figure 3: Microstructure of simulated CGHAZ. Scale bars: 20 µm.


Table 1: Parameters of simulated welding thermal cycling.


Figure 4: Microstructure of simulated HAZ with peak temperature at different temperature (VC=50C/s). Scale bars: 50 µm.


Figure 5: Cr23C6 in HAZ with Tm about 800C. Scale bas: 500 nm.

Carbide name: Cr23C6
Record No.: 1131
Carbide formula: Cr23C6
Carbide type: M23C6
Carbide composition in weight %: No data
Image type: LM, TEM
Steel name: 1Cr22Mn16N
Mat.No. (Wr.Nr.) designation: No data
DIN designation: No data
AISI/SAE/ASTM designation: No data
Other designation: No data
Steel group: High nitrogen steels
Steel composition in weight %: 0.148% C, 0.49% Si, 16.00% Mn, 22.07% Cr , 0.47% Ni, 0.56% N, 0.029% P, 0.002% S.
Heat treatment/condition: A Gleeble-1500 thermal simulation machine was used for experiment. The size of specimens for thermal simulation is 10.5 x 5.5 x 55 mm. Figure 2 shows the curve of the welding thermal cycling. There are four parameters in the curve, including heating rate (VH), peak temperature (Tm), staying time at peak temperature (tm) and the cooling rate (VC). The experiment divides into two aspects: the coarsegrained heat-affected zone (CGHAZ) was simulated under different welding conditions; different zones of the HAZ were simulated under same welding condition. Considering the fast heating and cooling of laser welding, the heating rate VH was 1350C/s in the simulated CGHAZ experiment. The peak temperature Tm is 1350C, the staying time at peak temperature tm was 0, and the cooling rate VC is 10, 30, 50, 100C/s and quenched, respectively. In the simulated different HAZ zone experiment, the heating rate VH was 1350C/s, and the staying time at peak temperature tm was 0, and the cooling rate VC is 50C/s, and the peak temperature Tm is 500, 800, 950, 1150 and 1350C, respectively. The parameters of simulated welding thermal cycling are shown in Table 1.
The specimens for optical observation were mechanically polished and etched with a 10% oxalic acid solution at 5 V for 30 s. Transmission electron microscopy (TEM) observation was carried out with a H-800 transmission electron microscope. The size of specimens used for the charpy Vnotched impact test of the simulated HAZ and base metal was 10 mm width x 5 mm height x 55 mm length in dimension with 2 mm V-notch. The impact test temperature was -40C.
Note: In order to obtain some information on the weldability of high nitrogen steel (HNS), the microstructure and mechanical properties of the heat-affected zone (HAZ) of HNS under laser welding conditions were investigated by using thermo-simulation technique. The experimental results indicate that the microstructure in the simulated HAZ of HNS consists of austenite and delta -ferrite that occurs in the grain boundary of austenite. The hardness of the simulated coarse-grained heat-affected zone (CGHAZ) increases when the cooling rate increases, and that of the simulated HAZ decreases while the peak temperature decreases. The results also show that the hardness of the simulated HAZ is higher than that of the base metal, indicating no softening of the HAZ under appropriate welding conditions. The impact toughness of simulated CGHAZ increases at first and then decreases with the increase of the cooling rate, whereas two brittle zones exist in the HAZ.

The microstructure of the steel is austenite with a small quantity of delta -ferrite, as shown in Fig. 1.

Figure 3 shows the microstructure of simulated CGHAZ. Figure 4 shows the microstructure of simulated different HAZ zone. It can be found that the microstructure consists of austenite and a small quantity of delta -ferrite that occurs in the grain boundary of austenite. With TEM observation, stacking faults and a high dislocation density are observed in the twinned sub-structure of austenite, and delta -ferrite also consisting of a large amount of dislocation appears polygonal. From Fig. 3 it can also be observed that the grain size of the simulated CGHAZ is increased with the decrease of VC. The grain size is about 45 µm as VC is 10C/s, whereas it is about 27.5 in the twinned sub-structure of austenite, and delta-ferrite also consisting of a large amount of dislocation appears polygonal. From Fig. 3 it can also be observed that the grain size of the simulated CGHAZ is increased with the decrease of VC. The grain size is about 45 µm as VC is 10C/s, whereas it is about 27.5 µm under quenched condition.m under quenched condition.

Some precipitates exist in the grain boundary when Tm is about 800C. By using TEM, the precipitates are identified to be Cr23C6, as shown in Fig. 5. Some papers declare that nitride precipitates occur in the base metal and weld HAZ of HNS. The precipitations in nickel-free high nitrogen steels, as function of C/N ratio, were also found in steels with a series of compositions: (0.06 to 0.32) C, (0.43 to 0.80) N, (17.7 to 22.1) Cr, (13.8 to 18.5) Mn. When C/N ratio is less than 0.1, aging at all temperatures produces only Cr2N precipitates. When the ratio is greater than 0.3, aging above 500C produces Cr23C6 and Cr2N precipitates and aging below 500C produces only Cr23C6 precipitates. When the ratio is between 0.1 and 0.3, aging above 500C produces only Cr2N precipitates and below 500C produces Cr23C6 and Cr2N precipitates. C/N ratio of HNS used in this paper is 0.26, and according to the above conclusions, Cr2N may occur in the HAZ of the HNS. However, nitride is not found with careful observation in this experiment. In the research on precipitation in high nitrogen steel 1Cr22Mn15N, nitride is not detected yet. Two reasons are suggested: 1) Carbon content of the steel reaches 0.15%, and high carbon content can promote the precipitation of carbide; 2) Mn, the element which reduces carbon solubility, is about 16% in the steel, and it increases the activity and diffuse speed of carbon. As a result, the reciprocity of both evidently accelerates the precipitation of Cr23C6. In addition, laser welding is a rapid heating and cooling process. So Cr2N does not occur in laser welding of HNS.
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