Effect of Ni and Mn on the Mechanical Properties of 22Cr Micro-duplex Stainless Steel

doi:10.1007/s40195-014-0162-z

Abstract

Abstract:

The tensile properties of 22Cr-2Ni-4Mn-0.2N micro-duplex stainless steels with different Ni and Mn contents were investigated. Duplex stainless steels were vacuum induction melted and hot rolled, then annealed at 1,000-1,100°C, at which temperature both the austenite and ferrite phases were stable. The volume fraction of the ferrite phase was markedly affected by the alloying elements of Mn and Ni; 1 wt% of Mn was equivalent to 0.4 wt% of Ni. All of the steels tested at room temperature showed the common strain-hardening behavior, while the steels tested at lower temperatures (-30 or -50°C) showed a distinct inflection point in their stress-strain curves, which resulted from the transformation of the austenite to strain-induced martensite. The onset strain (ε₀) of the inflection point in the stress-strain curve depended on the M _d30 value of the steel. Testing at lower temperatures resulted in smaller ε₀ and consequently higher strengths and fracture strains ( _f). The tensile behavior was examined from the perspective of austenite stability of the micro-duplex stainless steels with the different Ni and Mn contents.

Key words: Duplex stainless steel, Microstructure, Mechanical property, Alloy composition

Jun-Young Park, Yong-Sik Ahn. Effect of Ni and Mn on the Mechanical Properties of 22Cr Micro-duplex Stainless Steel[J]. Acta Metallurgica Sinica (English Letters), 2015, 28(1): 32-38.

Figures/Tables 11

Table 1 Chemical compositions of the duplex stainless steels

Alloy No.	Specimen	Mn	Ni	N	Cr	C	Mo	Si	Cu	Fe
1	4Mn-2Ni	4.07	1.97	0.19	21.51	0.032	0.302	0.99	0.8	Bal.
2	5Mn-1.5Ni	5.03	1.5	0.201	21.47	0.031	0.307	1.01	0.81	Bal.
3	6Mn-1Ni	5.94	1.03	0.204	21.5	0.028	0.3	1	0.8	Bal.
4	7Mn-0.5Ni	7	0.493	0.209	21.58	0.027	0.308	0.99	0.8	Bal.
5	8Mn	7.81	0	0.209	21.3	0.026	0.297	0.98	0.79	Bal.

Fig. 1 Optical micrograph of 4Mn-2Ni duplex stainless steel after annealing at 1,000°C

Fig. 2 Effect of annealing temperature on the volume fraction of the ferrite phase in various duplex stainless steels

Fig. 3 True tensile stress-strain curves at room temperature

Fig. 4 Comparison of true tensile stress-strain curves of 5Mn-1.5Ni steel at various temperatures

Fig. 5 True tensile stress-strain curves at -30°C a and -50°C b

Table 2 Inflection point of the tensile stress-strain curves at -30 and -50°C

Alloy No.	Strain at inflection ε ₀ (%)		Yield point (MPa)		Fracture strain ε _f (%)
Alloy No.	-30°C	-50°C	-30°C	-50°C	-30°C	-50°C
1	21.5	17.4	519.8	611.5	32.7	39.8
2	28.7	20.6	618.1	643.0	58.0	56.1
3	29.8	22.6	586.6	710.9	54.9	49.6
4	35.6	26.2	619.8	621.2	51.8	54.9
5	29.9	22.2	609.8	749.0	49.9	47.9

Fig. 6 XRD results of the as-received 8Mn steel and the steel after tensile testing at various temperatures

Table 3 Calculated M d30 values and martensite phase ratios

Alloy No.	M _d30	Volume fraction of SIM f _SIM (%)
Alloy No.	M _d30	-30°C	-50°C
1	-50.6	87.1 ± 0.6	86.8 ± 1.1
2	-58.3	78.9 ± 1.5	88.7 ± 0.6
3	-61.4	55.2 ± 1.1	78.2 ± 0.5
4	-67.9	37.2 ± 2.9	65.6 ± 1.5
5	-60.2	40.6 ± 1.7	65.0 ± 2.4

Fig. 7 Relationship between the onset strain (ε 0) of the inflection point and M d30

Fig. 8 TEM images of 8Mn steel after tensile testing at -50°C showing: a ferrite, b austenite containing martensite, c the austenite/ferrite-boundary region

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