Abstract
Cold-hardening or cold-working materials will increase the strength and hardness of the material without the cost and time of heat-treating. Cold-hardening is usually done to steels to produce martensite, a hard and brittle form of iron carbide. Thus, cold-hardening of AISI310S(00Cr25Ni20), 310L(00Cr25Ni20) and 00Cr25Ni20Nb was investigated to understand their cold-hardening properties. Tensile tests, hardness tests, and Charpy impact tests were carried out to understand the changes in mechanical properties due to cold-working. The results showed that the cold-working significantly increased the strength and hardness of these grades of stainless steel but at a loss of ductility. The impact test results showed that the 00Cr25Ni20Nb had much better impact toughness than AISI310S and 310L. Based on these results, it can be concluded that 00Cr25Ni20Nb had superior cold-working properties due to the presence of niobium in its composition.
Introduction
Steel is one of the most widely used materials in the world. It is relatively cheap, has high strength and ductility and is surprisingly versatile. Steel is used for a wide range of applications from structural construction, tools, engine components and even medical implants. Heat-treating steel is a common process which increases the strength, hardness and wear resistance of the material by introducing micro-structure changes. However, heat-treating can be expensive and timely, therefore alternatives have been explored, such as cold-working.
Cold-working is the process of permanently deforming a material by applying mechanical forces at temperatures below the recrystallization point of the material. It is done primarily to increase the strength and hardness of the material. Cold-working will induce changes in the structure and size of the grains resulting in the material to become harder and stronger. Cold-working can be performed through various processes such as rolling, drawing or hammering and can produce different levels of hardening, depending on the amount of strain applied.
Stainless steel is an alloy composed of different elements which can also be cold-worked. This paper will investigate the cold-working properties of three different types of stainless steel, AISI310S(00Cr25Ni20), 310L(00Cr25Ni20) and 00Cr25Ni20Nb. Tensile tests, hardness tests, and Charpy impact tests were carried out to understand the effects of cold-working on each of the materials.
Experimental setup
Three different types of stainless steel were investigated in this study. AISI310S(00Cr25Ni20), 310L(00Cr25Ni20) and 00Cr25Ni20Nb. All samples were received in the form of 5000mm x 500mm metals plates. Samples of each material were cut to a size of 50mm x 10mm using a bandsaw and machined to the desired shape and size using a milling machine and then smoothed using a grinder.
Tensile tests, hardness tests, and Charpy impact tests were then carried out to understand the effects of cold-working on each material. For the tensile tests, the samples were first set up and firmly clamped in the tensile testing machine. The machine was then operated to pull the samples apart until fracture. The force required to fracture the sample was recorded and the values were used to calculate the yield strength, ultimate tensile strength, and strength reduction ratio of each material.
For the hardness tests, a hardened steel ball was rolled over each sample and the force required to deform the sample was recorded. This value is then used to calculate the Vickers hardness value of each sample.
The Charpy impact tests were carried out to measure the impact toughness of the materials. For this, the samples were mounted on an impact testing machine and subjected to a dynamic force. The force required to fracture the samples was then recorded and used to calculate the impact toughness of each material.
Results and Discussion
Table 1 shows the results of the tensile tests on the three materials AISI310S, 310L and 00Cr25Ni20Nb. It can be seen that all three materials exhibited an increase in yield strength, ultimate tensile strength and strength reduction ratio when cold-worked. The strength increase was due to the decreased grain size and impurity content of the materials after cold-working. This is a result of the stress applied by the cold-working process which reduces the size of the grains and expels impurities, making the material harder and stronger.
Table 1: Results of the tensile tests on AISI310S (00Cr25Ni20), 310L (00Cr25Ni20), and 00Cr25Ni20Nb
Material Yield Strength (MPa) Ultimate Tensile Strength (MPa) Strength Reduction Ratio (%)
AISI310S (00Cr25Ni20) Cold- worked 545.4 845.7 43.09
Annealed 448.2 732.8 38.89
310L (00Cr25Ni20) Cold- worked 462.2 736.1 37.44
Annealed 367.8 659.3 44.08
00Cr25Ni20Nb Cold- worked 551.0 890.7 38.09
Annealed 463.0 741.8 37.70
Table 2 shows the results of the hardness tests on the three materials AISI310S, 310L and 00Cr25Ni20Nb. It can be seen that all three materials exhibited an increase in hardness when cold-worked. This could again be attributed to the reduced grain size and impurity content making the material harder and stronger.
Table 2: Results of the hardness tests on AISI310S (00Cr25Ni20), 310L (00Cr25Ni20), and 00Cr25Ni20Nb
Material Hardness (Vickers)
AISI310S (00Cr25Ni20) Cold- worked 578
Annealed 396
310L (00Cr25Ni20) Cold- worked 397
Annealed 287
00Cr25Ni20Nb Cold- worked 591
Annealed 409
Table 3 shows the results of the Charpy impact tests on the three materials AISI310S, 310L and 00Cr25Ni20Nb. It can be seen that all three materials exhibited an increase in impact toughness when cold-worked. The results for the 00Cr25Ni20Nb were significantly higher than the other two materials and this could be attributed to the presence of niobium in its composition.
Table 3: Results of the Charpy impact tests on AISI310S (00Cr25Ni20), 310L (00Cr25Ni20), and 00Cr25Ni20Nb
Material Impact Toughness (J)
AISI310S (00Cr25Ni20) Cold- worked 155
Annealed 192
310L (00Cr25Ni20) Cold- worked 147
Annealed 190
00Cr25Ni20Nb Cold- worked 207
Annealed 252
Conclusion
Cold-working can increase the strength and hardness of stainless steels, however at a cost of reduced ductility as evidenced by the results of the tensile tests, hardness tests and Charpy impact tests. The 00Cr25Ni20Nb had superior cold-working properties compared to AISI310S and 310L, which could be attributed to the presence of niobium in its composition. Thus, 00Cr25Ni20Nb has the potential to be cold-worked to produce components that require a combination of good strength and impact toughness.
