21º CBECIMAT - Congresso Brasileiro de Engenharia e Ciência dos Materiais 09 a 13 de Novembro de 2014, Cuiabá, MT, Brasil

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1 Comparative study between the mechanical formability of Mar300 and Mar350 steels Nathanael Wagner Sales Morais 1, a, Antônio Henrique das Virgens Neto 2,b and Hamilton Ferreira Gomes de Abreu 2,c 1 Departamento de Engenharia Metalúrgica e de Materiais, Universidade de São Paulo 2 Departamento de Engenharia Metalúrgica e de Materiais, Universidade Federal do Ceará a nathanaelmorais@gmail.com, b henrique_neto_ja@hotmail.com, c hamilton@ufc.br Keywords: Maraging 350, Maraging 300, Texture, Anisotropy Abstract. Abstract. The maraging steels have a great application on strategic industries like nuclear and aerospace. This paper evaluated the influence of post-annealing cooling rate and the influence of cold rolling level in the microstructure, hardness, texture and the influence in mechanical formability through simulation, of maraging 300 and maraging 350 steels. The cooling rate doesn t appears have any influence in microstructure, hardness or mechanical anisotropy. The cold rolling level demonstrated to be a high influence over the texture and anisotropy of maraging 300 steel and a higher on maraging 350 steel since this steel suffer a competition between recrystalization and precipitation for the most deformed condition. It was found too that maraging 350 steel have a better mechanical formability than maraging 300 steel. Introduction The maraging steels have Ni-Co-Mo-Ti quaternary base and can combine reasonable toughness with high mechanical strength. In the solution annealed state they present a body-centered cubic martensite due to the low carbon content, are extremely ductile and reasonably cold workable. After the aging heat treatment the material acquires an excellent mechanical strength. They are classified according to their tension strength (in ksi) into classes 200, 250, 300 and 350. These steels are used in applications ranging from defense industry, nuclear pressure vessels to the sports industry. [1]. Because his applications demands the aging, the mechanical behavior of maraging steels in the annealed conditions are less known and there are few papers describing it, in special, the dependence of his mechanical and plastic properties in correlation with the crystallographic texture. Hosoya et al [2] demonstrated that the maraging 350 after severe cold rolling develops a similar deformation texture of others BCC materials. Abreu et al [3] showed that a posterior annealing after the cold rolling partially destroys the deformation texture. Due the lack of information about the plastic anisotropy of the maraging steels, this paper studied how the plastic deformation levels and the post-annealing cooling rate modify the crystallographic texture and his effect in mechanical and plastic properties. One way to study the anisotropy is through the Lankford1s coefficients. These coefficients can be experimentally measured and can be simulated through texture measurements using the harmonic coefficients of the ODF. In this work, both methods were used to study these materials. For comparing the materials, were used the planar anisotropy coefficient and the normal anisotropy coefficient. Experimental Procedure This paper used maraging 300 and maraging 350 steels with chemical composition shown in Table 1. Both materials were received in hot-forged (900 C) condition in disks with 30cm diameter and 11mm of height. 5480

2 Table 1 Chemical Composition of maraging 300 and maraging 350 used in this paper. Material Ni Co Ti Mo Al C Fe Maraging ,68 9,62 0,86 4,85 0,28 <0,003 Bal Maraging ,19 12,17 1,38 4,84 0,38 <0,003 Bal The disks were machined to 2 coupons of 20cm of length, 10cm width and 11mm in height. The coupons were cold rolled in a laboratorial roller to 50%, 70% and 80% of original height, getting a piece of each thickness reduction of the coupon. After the cold rolling, the pieces of original coupon were annealed at 900 C by 1h and then cooled at furnace, at water and at air. Due the lack of material, just the maraging 350 in condition furnace cooled was mechanically tested, the other conditions were just simulated through the Popla LANK.exe routine. Mechanical test was used to validate the use of LANK.exe as a method to compare the anisotropy coefficients for both materials. Samples were prepared for metallographic, texture measurement, micro-hardness measurement and also tensile test. The Table 2 contains the samples description. Table 2 Sample Description Maraging 300 Maraging 350 Condition Cooling Code Condition Cooling Code Cold Rolled Water M300R50W Cold Rolled Water M350R50W 50% Air M300R50A 50% Air M350R50A Cold Rolled 70% Cold rolled 80% Furnace M300R50F Furnace M350R50F Water M300R70W Cold Rolled Water M350R70W Air M300R70A 70% Air M350R70A Furnace M300R70F Furnace M350R70F Water M300R80W Cold rolled Water M350R80W Air M300R80A 80% Air M350R80A Furnace M300R80F Furnace M350R80F The texture measurements were run in a X PERT PRO diffractometer, using Co radiation (λ = 1,789μm) for the (110), (200) and (211) pole figures. Measurements were computed with the help of two softwares: PopLA for simulating the Lankford s coefficients and MTEX to compare textures. The tensile tests were performed in an EMIC DL20000 machine using a deformation speed of 1mm/s and the lateral and thickness deformations were measured using a Zaas caliper (0,05mm precision). Results To validate the LANK.exe routine the measured results were compared to the simulated ones. The LANK.exe routine uses the coefficients of harmonic series obtained from Popla in calculation the Orientation Distribution Function (ODF). The measured results and simulated ones are show in Table 3 and the normal and planar anisotropy coefficients are shown in Table 4. Table 3 Comparison between the measured and simulated Lankford s coefficients through tensile test and LANK.exe Popla routine. Sample R0 Meas. R0 Simul. R45 Meas. R45 Simul. R90 Meas. R90 Simul. M350R50F 1,03 0,8 0,8 1,11 0,98 0,79 M350R70F 1,12 0,82 1,17 0,87 1,2 0,85 M350R80F 1,7 1,39 1,1 1,34 1,22 1,

3 Table 4 Comparison between the measured and simulated normal and planar anisotropy coefficients Sample ΔR Meas. ΔR Simul. Rm Meas. Rm Simul. M350R50F -0,005-0,105 0,798 1,058 M350R70F 1,17 0,87 1,12 0,82 M350R80F 1,1 1,34 1,7 1,39 There are a significant numeric difference between the measured values and the simulated ones, but the differences could be due the caliper imprecision. The values are different but the tendencies are similar, so the value cannot be compared but the tendency can. Influence of cooling rate on microstructure, texture and comparative anisotropy Due the non-diffusional character of martensitic transformation, isn t expected to have any difference between the microstructure or texture of these materials. In Figure 1 is shown the micrographs and the (110) pole figure of maraging 300 and maraging 350 after cold rolling 50% in 3 post-annealing different cooling rates. Figure 1 Micrographic analysis and (110) raw pole figure for maraging 300 and maraging 350. The samples are: a)m300r50w, b) M300R50A, c) M300R50F, d) M350R50W, e) M350R50A and f) M350R50F. As predicted by Abreu et al [3] the annealing destroyed the deformation texture in both materials and the cooling rate causes no influence in microstructure and texture, the micro-hardness measurements, shown on Table 5, doesn t show a significant difference in any of the 2 materials. Table 5 Micro-hardness measurements for maraging 300 and maraging 350 cited above. Maraging 300 HV1 10s ± SD Maraging 350 HV1 10s ± SD M300R50W 300 ± 4.5 M350R50W 320 ± 3.2 M300R50A 315 ± 6.0 M350R50A 325 ± 5.0 M300R50F 325 ± 7.0 M350R50F 327 ±

4 Since the texture was unaffected by the cooling rate, is expected no influence on anisotropy coefficients in both materials, this fact can be seen trough the Lankford s coefficients as in the normal and planar anisotropy coefficients listed in Table 6. Table 6 Simulated Lankford s coefficients for maraging 300 and maraging 350 cold rolled and annealed at 900 C by 1h cooled in different ways. Sample R0 R45 R90 Rm ΔR Sample R0 R45 R90 Rm ΔR M300R50W M350R50W M300R50A M350R50A M300R50F , M350R50F These results leads to see that the cooling rate is not a important parameter in annealing of maraging 300 and maraging 350 processing, since the mechanical properties, the microstructure and the simulated mechanical behavior are similar in all conditions studied in this paper. Influence of cold rolling on microstructure and texture and comparative anisotropy For comparing the influence of cold rolling, the furnace cooled samples were used. The level of cold deformation can make the deformation texture stronger or weaker. In BCC materials, the deformation characteristic texture can be seen on (110) pole figure and in the section PHI2 = 45 degree of the ODF in strengthening of the alpha fiber and cube-on-edge texture [needs reference]. Since Hosoya et al [2] showed that maraging cold rolling texture is similar of others BCC metals this paper looks just for annealing texture on maraging 300 and maraging 350 steels. Each material presented a different behavior when deformed at different levels, the maraging 300 steel presented first a softening and after a hardening due the grain growth and posterior refinement. The texture of this material has shows significative differences between the conditions, getting stronger by the deformation increases. Another fact noted was a competition between the recrystalization and a intermetallic precipitation in most deformed condition of maraging 350. These facts can be seen in Figure

5 Figure 2 Micrographic analysis and (110) raw pole figure for maraging 300 and maraging 350. The samples are: a)m300r50f, b) M300R70F, c) M300R80F, d) M350R50F, e) M350R70F and f) M350R80F. In accordance to micrographic analysis, were first there is a coarsement for both materials, the measured hardness slightly decreases and for most deformed condition, but the hardness increases in maraging 350 due precipitations. It can be seen on Table 7. Table 7 Micro-hardness measurements for maraging 300 and maraging 350 cited above. Maraging 300 HV1 10s ± SD Maraging 350 HV1 10s ± SD M300R50F 320 ± 7.0 M350R50F 325 ± 3.5 M300R70F 318 ± 3.0 M350R70F 314 ± 8.7 M300R80F 316 ± 15.0 M350R80F 400 ± 4.0 In other BCC metallic alloys, there is a similar competition between the recrystalization and precipitation, the martensite of maraging steels has a great dislocation density and naturally provides the pipe diffusion [need reference here] and the cold rolling increased the dislocation density even more, so it possible that the energy to recrystallize the grains was used in combination with the dislocations to accelerate the precipitation. Great blocks of dislocation can provide a quicker path to intermetallic precipitation. Due the morphological similarity, the intermetallic is believed to be a Laves phase. Since the texture was severely affected by deformation, the anisotropy coefficients was affected too since they are calculated based on texture. The Table 8 shows the simulated Lankford s coefficients for both steels in a fixed cooling rate. 5484

6 Table 8 Simulated Lankford s coefficients for maraging 300 and maraging 350 cold rolled at different levels and annealed at 900 C by 1h cooled in furnace. Sample R0 R45 R90 Rm ΔR Sample R0 R45 R90 Rm ΔR M300R50F M350R50F M300R70F M350R70F M300R80F , M350R80F For both materials, the Lankford s coefficients increase with the deformation, it happens due a favorable texture. The data shows too that for the conditions used in this work, the maraging 350 steel has a better formability than the maraging 300 due the higher normal anisotropy coefficients and a more stable planar anisotropy coefficient, References [1] MAGNEÉ A., DRAPIER J.M., DUMONT J., COUTSORADIS D., HABRAKENL., Cobalt containg High-Strengh Steels. Bruxelles Belgique: Centre Du Informacion du Cobalt, p [2] Hosoya Y, Shima Y, Ohkita T, Nishimoto A. Texture formation and aging behavior in 18% nickel maraging steel cold rolled and austenitized by simulated continuous annealing process. Trans ISIJ 1986;26:798. [3] Abreu, H.F.G., Tavares, S.S.M., Silva, J.J.M., Meneses, J.W.A., Bruno, A.D. ; The influence of an intermediate austenitization heat treatment in the texture of cold-rolled and aged 18% Ni maraging steel. Materials Characterization v.52, 2004 [4] Lopes, D.A., Restivo, T.A.G., Padilha, A.F.. Mechanical and thermal behaviour of U-Mo and U- Nb-Zr Alloys. Journal of Nuclear Materials 440, [5] Hölscher, M., Raabe, D,., Lücke, K, Steel Research, Rolling and recrystallization textures of bcc steels 62,v 12, (1991) [6] Raabe, D., Lücke, K.. Annealing textures of bcc metals. Scripta Met. 27, (1992). 5485