High strength and ductility of 34CrNiMo6 steel produced by laser solid forming

doi:10.1016/j.jmst.2018.09.062

Abstract

Abstract:

Because of the excellent mechanical properties of 34CrNiMo6 steel, it is widely used in high-value components. Many conventional approaches to strengthening-steels typically involve the loss of useful ductility. In this study, 34CrNiMo6 Steel having high strength and ductility is produced by laser solid forming (LSF) with a quenching-tempering (QT) treatment. Tempering of bainite is mainly by solid phase transformation in the previous LSF layers during the LSF process. The stable microstructure of LSF consists of ferrite and fine carbides. The microstructure transfers to tempered sorbite after heat-treatment. The tensile properties of the LSF steel meet those of the wrought standard. The UTS and elongation of LSF sample at 858 MPa, 19.2%, respectively, are greater than those of the wrought. The QT treatment enhanced the ultimate tensile strength and yield strength of the LSF sample. The ultimate tensile strength, yield strength, reduction in area, and elongation of the LSF+QT sample at 980 MPa, 916 MPa, 58.9%, and 13.9%, respectively, are greater than those of the wrought standard. The yield strength of the LSF+QT sample is approximately 1.27 times that of the wrought. The LSF samples failed in a ductile fracture mode, while the LSF+QT samples showed mixed-mode failure. The defects have only a small effect on the tensile properties owing to the excellent ductility of the LSF sample.

Key words: Laser solid forming, High strength and ductility, Quenching and tempering steel, Microstructure, Mechanical property

Chunping Huang, Xin Lin, Fencheng Liu, Haiou Yang, Weidong Huang. High strength and ductility of 34CrNiMo6 steel produced by laser solid forming[J]. J. Mater. Sci. Technol., 2019, 35(2): 377-387.

Figures/Tables 19

Fig. 1. SEM image of the as - received 34CrNiMo6 powder.

Table 1 Alloying compositions of 34CrNiMo6 powder (wt%).

C	Cr	Ni	Mo	Mn	Si	Fe
0.34	1.5	1.5	0.25	0.50	0.40	Balance

Table 2 Processing parameters of laser solid forming for 34CrNiMo6 steel.

Laser powder (W)	Spot diameter (mm)	Increment of axis-Z (mm)	Overlap (%)	Scanning speed (mm min^-1)	Powder feeding rate (g min^-1)
2800	2.5	0.2	45	600	10-15

Fig. 2. Geometry of the tensile testing specimen (in mm).

Fig. 3. Cross section of the top region in LSF block.

Fig. 4. OM micrographs of the top region of LSF block shown in Fig. 1: (a)-(f), corresponding to layer N through layer N-5.

Fig. 5. SEM micrograph of 34CrNiMo6 LSF block, (a)-(f) are corresponding to layer N through layer N-5.

Fig. 6. Micrograph of the middle region of the LSF block: (a) optical micrograph, (b) SEM image.

Fig. 7. Continuous cooling transformation (CCT) diagram of 34CrNiMo6 steel.

Fig. 8. Model showing schematically the formation of bainitic lath: (a) three dimensions, (b) two dimensions and (c) two dimensions for meta-bainite [20].

Fig. 9. FE-SEM image showing superledges and bainite of the last LSF layer.

Fig. 10. Typical microstructural features of QT heat treatment condition: (a) LSF + QT sample and (b) wrought substrate.

Fig. 11. Microhardness distribution of: (a) single laser cladding and (b) top region of LSF block.

Fig. 12. Microhardness of different condition samples.

Table 3 Tensile properties of the wrought 34CrNiMo6 steel.

Literature	UTS (MPa)	YS (MPa)	Elongation (%)	Reduction of area (%)
B. Zhang et al. [26]	842	719	19	55
R. Pippan et al. [27]	672	530	-	-
Wrought standard [28]	800-950	Min.600	Min.12	Min.55

Fig. 13. Tensile properties of different samples: (a) strength and (b) ductility.

Fig. 14. Fracture morphology of different condition sample: (a) and (b) LSF, (c) and (d) LSF + QT.

Fig. 15. Fracture morphology of LSF + porosity sample: (a) macro-fractography, (b) LoF particle, (c) dispersed porosity, (d) dimple.

Fig. 16. Fracture morphology of LSF + crack sample: (a) macro-fractography, (b) solidification crack, (c) dimple, (d) EDS results.

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