Laser forming repairing (LFR) technology is developed from the laser additive manufacturing, which has a high potential in high strength steel structures' repairing. 300M steel has been widely used in aviation and aerospace vehicles, to provide a high strength for aircraft landing gear and high strength bolts components, which in turn leads to a quick damage due to the severe service environment. If these damaged components can be repaired rapidly, the considerable savings in materials and costs can be achieved. In this work, the microstructure and mechanical properties of the LFRed 300M steel have been investigated. Results showed that the LFRed area can be clearly divided into three areas: the substrate zone (SZ), heat affected zone (HAZ) and repaired zone (RZ). The SZ was consisted of the mixture of martensite, bainite and a small amount of retained austenite. The HAZ presented an uneven martensite. The RZ presented an obvious heterogeneous microstructure, and the bainite, the mixture of martensite and bainite, and tempered martensite from the top to the bottom. After heat treatment, the microstructure became uniform with mixed tempered martensite and bainite. The tensile strength of the as-deposited LFRed 300M steel was far lower than those of the substrate. Its tensile strength and yield strength were 1459 MPa and 1163 MPa, respectively. After heat treatment, tensile strength (1965 MPa), yield strength (1653 MPa), elongation (11.7%) and reduction of area (38.4%) increased significantly and reached the same level of the substrate. Furthermore, compared to the as-deposited sample, the local strain of the RZ increased to 53% after heat treatment, and an obvious necking and breaking up happened as well. The strain hardening exponent of SZ and RZ were 0.1548 and 0.1138, which could be closely related to the compatible deformation capability.
Fund: Supported by National Natural Science Foundation of China (Nos.51323008, 51475380 and 51501154) and Program of Introducing Talents of Discipline to Universities of China (No.08040)
Fig.1 Schematics of the intercepting (a) and processing (b) of tensile samples (unit: mm, RZ—repaired zone, SZ—substrate zone)
Fig.2 Microstructures of different areas in as-deposited laser forming repaired (LFRed) 300M steel (Insets show the changes of content of bainite) (a) top of the RZ (b) middle-upper of the RZ (c) middle-lower of the RZ (d) bottom of the RZ (e) heat affected zone (HAZ) (f) SZ
Fig.3 Schematics of temperature field evolution of different areas in as-deposited samples (a) and microstructures' formation (b) (M—martensite, B—bainite, Bl—lower bainite, Bu—upper bainite)
Fig.4 XRD spectra of different areas in heat-treated LFRed 300M steel
Fig.5 Microstructures of different areas in heat-treated LFRed 300M steel
(a) top of the RZ (b) middle-upper of the RZ (c) middle-lower of the RZ (d) bottom of the RZ (e) HAZ (f) SZ(g) low magnification microstructure of the RZ (h) low magnification microstructure of the SZ
Fig.6 Fractographies of LFRed 300M steel with different conditions (a) macro-fractography of as-deposited (b) high magnified image of zone 1 in Fig.6a, showing the crack initiation of as-deposited (Inset shows the low magnification of crack initiation region) (c) high magnified image of zone 2 in Fig.6a, showing the radial area of as-deposited (d) macro-fractography of heat-treated (e) high magnified image of zone 1 in Fig.6d, showing the crack initiation of heat-treated (f) high magnified image of zone 2 in Fig.6d, showing the fiber area of heat-treated
Fig.7 Stress-strain curve of the as-deposited tensile samples (a), strain distributions along the central line during tensile loading of the specimen (b), and the corresponding localized strain distributions for the same tensile specimen at S1, S2, S3, S4, S5 and S6 (c)
Fig.8 Stress-strain curve of the heat-treated tensile samples (a), strain distributions along the central line during tensile loading of the specimen (b), and the corresponding localized strain distributions for the same tensile specimen at S1, S2, S3, S4, S5 and S6 (c)
Sample
Tensile
Yield
Elongation
Reduction
Fracture location
strength
strength
%
of area
MPa
MPa
%
Forging standard
≥1925
≥1630
≥12.5
≥50.6
-
Substrate
1993
1624
12.1
41.2
-
As-deposited LFRed
1459±11
1163±73
5.8±0.8
14.6±0.3
Repaired zone
Heat-treated LFRed
1965±12
1653±4
11.7±0.6
38.4±3.2
Repaired zone
Table 1 Room temperature mechanical properties with different conditions of 300M steel
[1]
Youngblood J L, Raghavan M.Correlation of microstructure with mechanical properties of 300M steel[J]. Metall. Trans., 1977, 8A: 1439
[2]
Tomita Y, Okawa T.Effect of microstructure on mechanical properties of isothermally bainite-transformed 300M steel[J]. Mater. Sci. Eng., 1993, A172: 145
[3]
Zhang S S, Li M Q, Liu Y G, et al.The growth behavior of auste-nite grain in the heating process of 300M steel[J]. Mater. Sci. Eng., 2011, A528: 4967
[4]
Zhang H P, Wang C X, Du X.Aircraft landing gear with the development of 300M ultra high strength steel and research[J]. J. Harbin Univ. Sci. Technol., 2011, 16(6): 73
Gadag S P, Srinivasan M N, Mordike B L.Effect of laser processing parameters on the structure of ductile iron[J]. Mater. Sci. Eng., 1995, A196: 145
[7]
O?oro J, Ranninger C.Fatigue behaviour of laser welds of high-strength low-alloy steels[J]. J. Mater. Process. Technol., 1997, 68: 68
[8]
Kattire P, Paul S, Singh R, et al.Experimental characterization of laser cladding of CPM 9V on H13 tool steel for die repair applications[J]. J. Manuf. Process., 2015, 20: 492
[9]
Hu Y P, Chen C W, Mukherjee K.Development of a new laser cla-dding process for manufacturing cutting and stamping dies[J]. J. Mater. Sci., 1998, 33: 1287
[10]
Leunda J, Soriano C, Sanz C, et al.Laser cladding of vanadium-carbide tool steels for die repair[J]. Phys. Procedia, 2011, 12: 345
[11]
Lin X, Cao Y Q, Wu X Y, et al.Microstructure and mechanical properties of laser forming repaired 17-4PH stainless steel[J]. Mater. Sci. Eng., 2012, A553: 80
[12]
Li L J.Repair of directionally solidified superalloy GTD-111 by laser-engineered net shaping[J]. J. Mater. Sci., 2006, 41: 7886
[13]
Zhang Z H, Lin P Y, Zhou H, et al.Microstructure, hardness, and thermal fatigue behavior of H21 steel processed by laser surface remelting[J]. Appl. Surf. Sci., 2013, 276: 62
[14]
Tong X, Dai M J, Zhang Z H.Thermal fatigue resistance of H13 steel treated by selective laser surface melting and CrNi alloying[J]. Appl. Surf. Sci., 2013, 271: 373
[15]
Xu Q D, Lin X, Song M H, et al.Microstructure of heat-affected zone of laser forming repaired 2Cr13 stainless steel[J]. Acta Metall. Sin., 2013, 49: 605
Liu Q, Wang Y D, Zheng H, et al.TC17 titanium alloy laser melting deposition repair process and properties[J]. Opt. Laser Technol., 2016, 82: 1
[17]
Dinda G P, Dasgupta A K, Mazumder J.Laser aided direct metal deposition of Inconel 625 superalloy: Microstructural evolution and thermal stability[J]. Mater. Sci. Eng., 2009, A509: 98
[18]
Cong D L, Zhou H, Ren Z N, et al.The thermal fatigue resistance of H13 steel repaired by a biomimetic laser remelting process[J]. Mater. Des., 2014, 55: 597
[19]
Sun S D, Liu Q C, Brandt M, et al.Effect of laser clad repair on the fatigue behaviour of ultra-high strength AISI 4340 steel[J]. Mater. Sci. Eng., 2014, A606: 46
[20]
Kang M K, Yang Y Q, Zhang X Y, et al.Bainitic transformations in silicon-containing steels[J]. Acta Metall. Sin., 1996, 32: 897
Wang Y D, Tang H B, Fang Y L, et al.Effect of heat treatment on microstructure and mechanical properties of laser melting deposited 1Cr12Ni2WMoVNb steel[J]. Mater. Sci. Eng., 2010, A528: 474
[22]
Zhang L, Zhang Y F, Huo L X, et al.Microstructure and properties of 30CrMnSiNi2A steel electron beam welded joints[J]. Trans. China Welding Inst., 2002, 23(1): 73
Liu F G, Li T J, Wang C X, et al.Effect of postweld heat treatment on microstructure and mechanical properties of 05Cr17Ni4Cu4Nb steel weld joint[J]. Heat Treat. Met., 2010, 35(11): 65
Qi C L.Effects of heat treatment on microstructure and mechanical properties of D406A steel welded joint [D]. Harbin: Harbin Institute of Technology, 2012
