Effect of Heat Treatment on Structure, Mechanics and Corrosion Resistance of 316L Austenitic Stainless Steel

Changing 316L austenitic stainless steel by heat treatment σ and δ volume fraction of phase can optimize the mechanical and corrosion properties of 316L austenitic stainless steel manufactured by gas metal arc additive (GMAAM). The results show that heat treatment at 1000-1200 ℃ for 1h has little effect on the grain morphology of steel, but σ、δ phase content has great influence. 1000 ℃ heat treatment effectively improves the σ ultimate strength and yield strength of the steel are increased, while the elastic modulus and yield strength are decreased. 1100-1200 ℃ heat treatment completely eliminates σ ultimate tensile strength and yield strength decrease, while the elastic modulus and yield strength increase. σ strengthening effect of phase is better than δ But it will reduce the plasticity of the steel and increase the possibility of cracks in the steel. At the same time, limit the σ phase harmony δ amount of phase can improve the corrosion resistance of steel. And δ In comparison, σ corrosion resistance of steel is more affected than that of steel.

0. Introduction

Austenitic stainless steel has a stable and complete austenitic structure. Because of its excellent corrosion resistance and sufficient high temperature mechanical properties, it has been widely used in chemical production, ship manufacturing, high temperature bolts, nuclear reactors and other modern industries, with good manufacturability and weldability. Additive manufacturing can directly manufacture and repair metal parts, reducing processing time and cost. Compared with traditional manufacturing technologies such as casting and forging, additive manufacturing has technical advantages and economic competitiveness, especially for the manufacturing and repair of large and complex metal components. Additive manufacturing has the characteristics of high temperature gradient, fast cooling speed, cyclic heating, etc. Compared with the traditional manufacturing process, it has a large difference in microstructure.
The phase transformation behavior and structure change of 316L austenitic stainless steel during heat treatment were studied, and the mechanical properties and corrosion properties of 316L austenitic stainless steel were optimized.

1. Experimental process

1.1 Microstructure characterization

Use the discharge wire to cut the metallographic specimen from the deposited part, and then conduct standard mechanical polishing and etching (4g Cu SO₄, 20mL HCl and 20mL H₂O). The microstructure of GMA-AM 316L was studied by Olympus BX51M optical microscope (OM) and JSM-6010 scanning electron microscope (SEM). At working distance of 3 μ In JEOL-JEM 7001F, electron backscattering diffraction (EB-SD) technology is used to identify the texture and orientation of the crystal, and the corresponding electron backscattering patterns are analyzed to identify the texture and orientation of the crystal. The acceleration voltage of SEM and EBSD imaging is 20k V. Image ProPlus software was used to calculate three SEM photos of the same sample in GMA-AM 316L as deposited and after heat treatment δ Phase harmony σ Volume fraction of the phase.

1.2 Tensile test

The 50k N-SANS electronic lighter is used, with the maximum load of 50k N and crosshead speed of 0.5mm/min. The tensile test is conducted according to ISO 6892-1:2009 standard, and the strain is measured with an electronic extensometer. In order to conduct tensile test at room temperature, round specimens with a diameter of 5 mm, a gauge length of 35 mm and a total length of 71 mm were prepared. The principal axis of the tensile specimen is parallel to the deposition direction. In order to reduce the measurement error, the average value of three samples under the same conditions was calculated. The fracture surface and cross section of the tensile specimen were observed by scanning electron microscopy (SEM).

1.3 Electrochemical experiment

Cut the sample for electrochemical test into 12mm × 12mm × 2mm as working electrode embedded in epoxy resin. The specimen is electrically contacted from the rear by spot welding a copper conductor on the back of the specimen. Use continuous grade silicon carbide sandpaper to grind the surface of each working electrode to 2000 # sand, and then use 0.5 μm Diamond plaster polishing. The non working surface of the electrode is sealed with silicone rubber. Electrochemical tests were carried out in 3.5wt% Na Cl solution at 25 ° C using the corresponding electrochemical workstation. Prepare the solution with deionized water and analytical grade chemicals. The electrochemical cell consists of three electrodes: counter electrode, reference electrode and working electrode. Platinum foil and saturated calomel electrode (SCE) were used as counting electrode and reference electrode respectively. All potentials quoted in this work refer to SCE. Before the sample is immersed, bubble nitrogen at a rate of 0.15 L/min for 30 min to reduce the oxygen content in the solution. Ventilation continued throughout the test. In order to obtain a stable state, the open circuit potential (Eo) was recorded for 30 min. At a given scanning speed (30m V/min), the action potential polarization measurement is carried out by sweeping the potential from 50m V below the stable open circuit potential until the current density exceeds 10-4A/cm2. The potential when the current density increases significantly is considered as the pitting potential (Ep). The corrosion current density of different samples was measured by Tafel extrapolation according to the potentiodynamic polarization curve.

2. Results

2.1 Microstructure

In as deposited steel, δ and σ Phase in γ matrix shows very fine wormlike morphology (Fig. 1 (a)). Because of the low carbon concentration, there is no carbide in the steel. Table 1 shows that among deposited steel and heat treated steel δ and σ Volume fraction (volume percent) of the phase. δ Phase harmony σ volume fraction (vol%) of the phase is 7.84 vol% and 4.45 vol%, respectively. When heat treated at 1000 ℃ for 1h (Fig. 1 (b)), σ Connecting δ It is formed by gradual replacement of, effectively without changing the original δ form of phase, but make the surplus δ Phase spheroidization. δ phase volume fraction decreased from 7.84vol% to 3.92vol%, σ phase volume fraction increased from 4.45 vol% to 6.98 vol%. When heat treated at 1100 ° C and 1200 ° C for 1h (Fig. 1 (c) and (d)), σ phase is completely dissolved in γ In the matrix. δ phase is granular, which decreases from 7.84vol% to 5.79vol% (1100 ° C/1h) and 3.96vol% (1200 ° C/1h) respectively. When heat treated at 1200 ℃ for 4h (Fig. 1 (e)), σ and δ phase is completely dissolved in the γ In the matrix.
Table.1 As deposited and heat treated GMA-AM 316L δ and σ Volume fraction of phase (volume percentage)

	σ	8
As deposited steel	7.84	4.45
1000℃/1h,WQ	3.92	6.98
1100℃/1h,WQ	5.79	0
1200℃/1h,WQ	3.96	0
1200℃/4h,WQ	0	0

However, it is difficult to distinguish the grains of steel from SEM images, so EBSD technology is adopted. Figure 2 shows the EBSD orientation image and inversion diagram of the deposited and heat treated steel. Each color corresponds to the orientation of austenite grains relative to the lattice, and each color corresponds to a unique combination of Euler angles. Therefore, grains with the same crystallographic orientation have similar colors. In as deposited steel, the grains are columnar (Fig. 2 (a)). After heat treatment at 1000 ℃ – 1200 ℃ for 1h, the columnar grain has no obvious change compared with the as deposited state (Fig. 2 (b) – (d)). Long holding time at 1200 ° C leads to grain coarsening and transformation from columnar to equiaxed, as shown in Fig. 2 (e).

Figure.1 GMA-AM 316L as deposited and heat treated

Figure.2 Orientation image of GMA-AM 316L as deposited and heat treated

2.2 Tensile properties

The ultimate and yield strength (YS) with error bars are calculated from the stress/strain curves (Fig. 3) for deposited and heat treated steels. The tensile strength and yield strength of the deposited steel are 533MPa and 235MPa respectively. After heat treatment at 1000 ℃ for 1h, the ultimate strength increases from 533MPa to 549MPa, the yield strength increases from 235MPa to 242MPa, the elongation decreases from 48% to 41%, and the reduction of area decreases from 64% to 61%. At 1100-1200 ℃, the ultimate strength and yield strength of heat treated steel are reduced to 474MPa and 204MPa respectively, while the ultimate strength and yield strength of heat treated steel are increased to 70% and 83% respectively. The mechanical properties of the deposited steel are equivalent to that of the solution treated 316L steel, but the ultimate tensile strength and yield strength of the steel are lower than those of the forged 316L steel when heat treated at 1100 ℃ and 1200 ℃, and the tensile properties of the deposited steel and heat treated steel are better than the industrial requirements of the forged 316L steel.
Fig. 4 (a) and (c) show the fracture surfaces of as deposited and heat-treated (1100 ° c/1h) steels, respectively. The reduction of area of heat treated (1100 ℃/1h) steel is larger than that of deposited steel. The fracture surface with high magnification (Fig. 4 (b) and (d)) shows the concave surface, which indicates that the fracture type of the deposited and heat-treated steel belongs to ductile fracture.

Figure.3 Stress Strain Curves of GMA-AM 316L Clad and Heat Treated Tensile Specimens

Figure.4 Fracture surface of GMA-AM 316L as deposited and heat treated (1100 ° C/1h, WQ)

2.3 Corrosion performance

The corrosion behavior of as deposited and heat-treated steel in 3.5% NaCl solution was studied by potentiodynamic polarization method. Figure 5 and Table 2 show the corresponding polarization curves and measured values of electrochemical parameters. The low corrosion current density (i corr) and the high difference (△ E) between pitting potential and corrosion potential indicate that the corrosion rate is slow and the corrosion resistance is high. According to Table 2, the low to high order of i corrosion is (1200 ° C/4h)<(1200 ° C/1h)<(1100 ° C/1h)<(1000 ° C/1h)<as deposited state, and the high to low order of △ E, indicating that the corrosion resistance of steel is from high to low order. The results show that heat treatment can improve the corrosion resistance of steel. The corrosion resistance of heat treated (1200 ℃/4h) steel is the best.

Figure.5 The potentiodynamic curves of GMA-AM 316L as deposited and heat-treated in 3.5% Na Cl solution at 25 ℃: (a) as deposited; （b)1000℃/1h,WQ; (C)1100℃/1h,WQ; (d)1200℃/1h,WQ; (e)1200℃/4h,WQ 
Table.2 Electrochemical parameters of GMA-AM 316L after deposition and heat treatment in 3.5% Na Cl solution at 25 ° C

	Corrosion current density icorr(10-8A/cm2)	Corrosion potential Ecorr/VSCE	Pitting potential Ep/VSCE	△E=Ep-Ecorr/VSCE
As deposited steel	7.83	-0.15	0.47	0.62
1000℃/1h,WQ	6.98	-0.15	0.57	0.72
1100℃/1h,WQ	5.31	-0.14	0.97	1.11
1200℃/1h,WQ	4.68	-0.16	1.02	1.18
1200℃/4h,WQ	4.45	-0.15	1.1	1.25

3. Conclusion

• 1) 1000 ℃ heat treatment effectively improves the σ content of phase makes the residual δ Phase spheroidization. Heat treatment at 1100 ℃ and 1200 ℃ for 1h, σ phase is completely dissolved in γ In the matrix, δ phase decreases with the increase of heat treatment temperature. After heat treatment at 1000-1200 ℃ for 1h δ Phase harmony σ re is no obvious change in columnar crystal due to the pinning effect of phase. The holding time at 1200 ℃ is long, and the grains change from columnar to equiaxed, δ and σ phase is completely dissolved.
• 2) σ increase of phase leads to the increase of ultimate tensile strength and yield strength of steel after 1000 ℃ heat treatment, while the yield strength and yield strength of steel after heat treatment decrease; 1100-1200 ℃ heat treatment completely eliminates σ ultimate tensile strength and yield strength decrease, while the yield strength and yield strength increase. The tensile properties of the deposited steel and heat treated steel exceed the industrial requirements of 316L steel. In steel σ strengthening effect of phase is better than δ Phase, but the effect on plasticity is greater. σ Phase increases the possibility of crack generation.
• 3) Heat treatment improves the corrosion resistance of steel. With the increase of heat treatment temperature and time, the corrosion resistance of steel increases. σ formation of phase leads to σ Cr deficiency area at the phase interface, thus increasing the corrosion sensitivity. And δ In comparison, σ corrosion resistance of steel is more affected than that of steel. The fully austenitic structure after heat treatment (1200 ° C/4h) has the best corrosion resistance.

Author: Dang Li

Source: 316L Forgings Supplier: www.epowermetals.com