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in Vol. 14 - January Issue - Year 2013
Effect Of Tempering On The Low Carbon Cast Steel Shot
Figure 1: The microstructures of the low-carbon cast steel shot samples

Figure 1: The microstructures of the low-carbon cast steel shot samples

Figure 2: The sample hardness after centrifugal atomizing and tempering at various temperatures have been applied

Figure 2: The sample hardness after centrifugal atomizing and tempering at various temperatures have been applied

Figure 3: The breakdown curve of the samples at various tempering temperatures

Figure 3: The breakdown curve of the samples at various tempering temperatures

Figure 4: The hardness and the ERVIN life at various tempering temperatures

Figure 4: The hardness and the ERVIN life at various tempering temperatures

Figure 5: The wearing morphology of the samples with the hardness of HRC 43 after centrifugal atomizing and tempering at 250

Figure 5: The wearing morphology of the samples with the hardness of HRC 43 after centrifugal atomizing and tempering at 250

Abstract: The low-carbon cast steel shot was obtained by the centrifugal atomizer and underwent a tempering process. The Vickers hardness tester, the optics microscope, X-ray diffraction and the ERVIN Test Machine are used to derive the hardness, metallograph, phase and ERVIN life of the low-carbon cast steel shot respectively. And the major conclusions are as follows: The hardness of low-carbon cast steel shot is dependent on the tempering temperature and decreases gradually with the raising of the (tempering) temperature; meanwhile, the ERVIN life decreases gradually with the raising of hardness.

1. Introduction

The as-cast low-carbon cast steel shot sample used in this experiment, manufactured by Shandong Kaitai Group, as shown in Table 1, is melted by using medium frequency induction furnace and formed by centrifugal atomizing method. The samples are in good spherical shape with the least amount of irregular particles. In the standards of ISO11124-4[1] and SAE J2175[2], it states that the microstructure of the low carbon cast steel shot abrasives shall have a bainitic or martensitic structure with the hardness of HRC 40-51 or HV 390-520. Using the standards as guideline, six various tempering temperatures ranging from 150ºC to 650ºC are chosen and then air-cooled. The processed samples are etched by the alcohol solution with 4% of nitric acid and then studied for microstructures with by metalloscopy.The micro-hardness of the processed sample is tested by using the Vickers hardness tester with a 500g load. The ERVIN life is represented by the breakdown curve, which is tested by an ERVIN test machine. The ERVIN life test procedure is performed as follows: placing placing 100g of the sample in the ERVIN test machine; running 500 cycles and then screening the samples through a sieve with a 0.6 mm diameter mesh; weighting the retained sample; and finally returning the remaining samples back to the ERVIN test machine to continue the next 500 cycles till less than 5% of the sample is left[3,4].

Chemical component (wt%)

C

Mn

Si

S

P

Sample contents

0.14

1.13

0. 38

0.046

0.018

Standard contents

0.08~0.20

0.35~1.50

0.1~2.0

<=0.05

<=0.05


Table 1: The chemical component of the as-cast low carbon cast steel shot and relevant ISO chemistry required

2. Test result and its Analysis

2.1 Microstructure

The microstructure of the as-cast low-carbon steel shot is the a - Fe ferrite and low-carbon lath martensite, which is transformed from austenite during the centrifugal atomizing process. This metallurgical structure exhibits carbon segregation, so its wearing resistance will be lower, as it is easily broken up and has a short life. As seen in Figure 1 (a), the as-cast low-carbon cast steel shot sample is tempered at 150ºC for 30 minutes; it shows that the lath martensite does not have a clear-cut boundary, as the carbon element diffuses. In Figure 1 (b), when the as-cast sample is tempered at 250ºC and preserved for 30 minutes, the microstructure shows that the a - Fe ferrite and low-carbon lath martensite disappears, and the substitute is troostite. In Figure 1 (c), the sample is tempered at 350ºC and the structure looks like the structure in Figure 1 (b), but just becomes more well-distributed. After the sample is tempered at 450ºC for 30 minutes {Figure 1 (d)}, the microstructure begins to transform where the low-carbon lath martensite has disappeared completely, but the microstructure is not homogeneous enough. When the as-cast low-carbon steel shot has been tempered at 550ºC and 650ºC {Figures 1 (e) and (f)}, the metallurgical structure of low-carbon cast steel shot is sorbite with small interplanar crystal spacing and is very homogeneous.

2.2 Microhardness

Figure 2 shows the hardness with the various tempering temperatures when the sample is tempered. As the tempering temperature increases, the hardness of the sample after centrifugal atomizer but no tempering decreases gradually from HRC45.8 to HRC20, which is the hardness of the sample tempering at 650ºC. In ISO 11124-4[1] and SAE J2175[2], the hardness of the low-carbon cast steel shot should be HRC40-51, so based on the curve in Figure 2, it is concluded that the tempering temperature after centrifugal atomization should be controlled between 0ºC and 300ºC. According to customer requests, they also could decide the tempering temperature to get the required hardness.

2.3 ERVIN Life

When evaluating the average ERVIN life by the area under the breakdown curve, the ERVIN life is equal to the sum of the areas divided by 100%[3,4], so the ERVIN life is proportional to the sum of the areas under the breakdown curve. Figure 3 shows that as the tempering temperature increases, the breakdown curve extends further. This implies that the sum of the area under the curve gets larger and the ERVIN life prolongs gradually. From Figure 3, it can also be found that the ERVIN life of the samples after centrifugal atomization and tempering is longer than the ERVIN life of the samples with only centrifugal atomization but no tempering, because the low-carbon lath martensite and the retained austenite in the sample transform into sorbite as the tempering temperature increases. The transformation from lath martensite to sorbite improves the sample resistance to impact and deformation capability. As a result, the ERVIN life of the samples will be longer.

Figure 4 shows the relationship between the hardness and the ERVIN life of the samples at various tempering temperatures. From the figure, it can be seen that at the 150ºC tempering temperature, the hardness of the sample changes a little, but the ERVIN life increases by 9.6%. In addition, the fastest increase rate ranges from 150ºC to 450ºC. Beyond this range, the rate of increase becomes slower, but it still increases.

Figure 5 shows the appearance of the sample at various cycles of ERVIN life test and the sifting of the sample. Harness of the sample is HRC43 (HV420) after centrifugal atomizer and tempering at 250ºC. A sieve with 0.6 mm diameter is used to separate the sample and the sifting of the sample. It is clear that the average diameter of the shot gradually decreases. Figure 5 (b) shows the sample after 400 cycles, all of the shots having become deformed into spherical shapes but have many surfaces after continual hits on the target of the ERVIN test machine, which has a hardness above HRC63. It also shows that a portion of samples with defects of porosity or micro-crack are broken down and then rounded again: this is a kind of ductile fracture. Figure 5 (d) shows that most of the sample is broken down and rounded. Figures 5 (e) to 5 (g) show that the shots reduce in size, but the wearing resistance is enhanced. Some shots with little defects can hold up to 2400 cycles; these shots show minimal reduction of the size with a compact and uniform microstructure.

2.4 Conclusion

The microstructure of the as-cast low-carbon cast steel shot is a - Fe ferrite and low carbon lath martensite. The microstructure will transform from the tempered martensite to the tempered troostite and tempered sorbite changes along with the raise in tempering temperature [5]. Tempering will reduce the microhardness of the low-carbon cast steel shot gradually as the tempering temperature increases. According to the statements regarding the low-carbon cast steel shot standard ISO11124-4[1] and SAE J2175[2], the hardness of the low-carbon cast steel shot should be HRC40-51, and the tempering temperature should be controlled between 0ºC and 300ºC to get the required hardness. According to customer requests, they also could decide the tempering temperature to get the required hardness in this study. After centrifugal atomizing and tempering, the ERVIN life of the low-carbon cast steel shot will increase as the tempering temperature rises. Tempering heat treatment can improve the durability and the wear resistance of the low-carbon cast steel shot.

2.5 References

1. ISO11124-4.1993, Specifications for metallic blast-cleaning abrasives Part 4: Low-carbon Cast-Steel Shot.
2. SAE J2175.1991-6, Specifications for low-carbon cast steel shot.
3. Ervin Industries. 2002, The Ervin Test Machine.
4. SAE J455APR96. January 1996, Metallic Shot And Grit Mechanical Testing.
5. Zhengfeng Qi. 1988.6, Heat Treatment Principles for Metals. Beijing,
6. Ruwei Liu. 1996.5, Analysis of Several Questions Associated with Abrasive for Shot-blast Cleaning of Castings .Foundry, No.5,
7. Ruwei Liu. 2004.2, Comparative Analysis of Abrasive for Shot Blasting Steel Structure. Steel Construction,Vol.19, No.71, pp.70-71.

Ruwei Liu, Xiangxiao Jiang, Hongju Gao
College of Material Science and
Engineering, Shandong University
Jinan 250061, P.R. of China

Ruwei Liu, Chengmin Wu, Ruiguo Wang
Shandong Kaitai Group
Zouping 256217, P.R. of China

Tel. contact of Ruwei Liu: +86.13616407190
E-mail: shotgrit@163.com