Metallographic diagram of 2Cr13 (spheroidizing annealing and tempering after 85 days colloidal storage tank corrosion resistance test)

Table 1 Chemical composition of experiment materials

Chemical component, wt%

C: 0.19

Si: 0.10

Mn: 0.23

P: 0.01

S: 0.01

Cr: 12.66

Ni: 0.06

V: 0.01

Cu: 0.11

Mo: 0.01

Nb: 0.01

Ti: 0.02

Al: 0.01

B: 0.01

Fe: bal

Figure 1 Microstructure of 2Cr13 before spheroidizing anneal

The proposed experiment aims to determine the corrosion resistance of a stainless steel alloy (2Cr13) with spheroidizing annealling and 85 days’ colloidal storage. 2Cr13 is a ferritic stainless steel alloy of Fe-Cr-Ni type, with a nominal Cr content of 13%. The chemical composition (Table 1) of the 2Cr13 accepted in the experiment was characterized by a high Cr content and small amounts of Ni, Mn, Si and C.

2Cr13 used in this experiment was based on hot-rolled steel to further heat treat. The ferrite microstructures of 2Cr13 were formed after spheroidizing annealing treatment at 860°C. The optical microscopy (Figure 1) of the samples showed the typical spheroidized ferrite grains with slightly coherent ferrite boundaries. After spheroidizing annealing, the sample was then cut into pieces, and stored in colloidal for 85 days.

The experiment was conducted by the saturated copper sulfate solution as the corrosive medium. The pieces of 2Cr13 sample were immersed for 18 hours and then taken out for corrosion test. After the immersion, it was found that the sample surface was still covered by a thin layer of oxide film. The thickness of this layer was measured by a profilometer.

In order to evaluate the anti-corrosive performance of the samples, the samples’ surface were examined by optical microscopy with the aid of a chromatic contrast plate before and after immersion. The results (Figure 2) demonstrated that before the immersion, some bright red patches were observed, because of the precipitates of Cr, Fe and Ni during the spheroidizing annealing process. After 18 hours’ immersion, the corrosion had generated a red-brown patina which covered the original bright red patches.

The corrosion rates of 2Cr13 samples were then calculated, based on the measured oxide film thickness. The calculated corrosion rates (Table 2) clearly showed that the corrosion resistance of 2Cr13 was improved after spheroidizing annealing and was further improved after 85 days’ colloidal storage, compared to the initial corrosion rate.

Table 2 Corrosion rate of 2Cr13

Corrosion rate (µm/year)

Before annealing: 0.5

After Spheroidizing Annealing: 0.1

After Colloidal Storage: 0.05

Figure 2 Optical micrographs of 2Cr13 before and after immersion

In conclusion, the experiment results show that 2Cr13 after spheroidizing annealing and 85 days’ colloidal storage has excellent corrosion resistance in the saturated copper sulfate solution. This indicates that spheroidized ferrite microstructure and the 85 days’ colloidal storage provide better anti-corrosive performance for 2Cr13. Therefore, the corrosion resistance of stainless steel alloy can be enhanced through spheroidizing annealing, and further enhanced by subsequent colloidal storage.

In metallography analysis of balance GB/T 12770-1991, the sample of stainless steel 304L2Cr13 is collected and examined by its ball annealed and tempered condition stored in jelly container for 85 days. The carbon steel we used has higher strength and permeability. The results of optical microscope study shows that the steel structure is uniform and its grains are equiaxed. Under the etchants of picral solution, the matrix of the steel showed austenite. According to the orientation measurement, the average grain size was calculated to be 38.9μm.

It is revealed that both the ferrite content and martensite content is higher than 50%. The components of the ball annealed and tempered condition are mainly austenite, a small part of martensite and ferrite. The coexistence of austenite and martensite allows the steel to possess higher strength and better corrosion resistance. The element transmission mapping of energy spectrum analysis shows that there exist some non-metallic inclusions in the steel, mainly including oxides and sulfides.

It can be concluded that the thermo-physical process contents proper temperature and holding time, so that the position of elements distribution and microstructure of 304L2Cr13 can be controlled effectively. By ball annealed and tempered condition, the uniform matrix of austenite and high strength and corrosion resistance can be achieved. The results also demonstrates the limit of steel performance is obeying the standards of GB/T 12770-1991. Owing to the careful process control, the steel samples of 304L2Cr13 has demonstrated excellent corrosion resistance performance and thus satisfying the end-course-use condition.