MATERIALS TRANSACTIONS
Online ISSN : 1347-5320
Print ISSN : 1345-9678
ISSN-L : 1345-9678
Influence of As-Cast Microstructure on the Impact Wear Resistance of 27 mass%Cr Cast Iron
Ngo Huynh Kinh LuanKoreaki KoizumiKuniaki MizunoYutaka YamadaTetsuya Okuyama
著者情報
ジャーナル フリー HTML

2019 年 60 巻 11 号 p. 2475-2480

詳細
Abstract

The impact wear resistance of 27 mass%Cr cast iron which as-cast microstructure influenced by cooling rate during solidification was investigated by using abrasive blasting machine. Hardness and volume fraction of martensite of samples with and without contacting with an iron chiller were measured before and after abrasive blasting test. Refinement of microstructure which was due to increase of cooling rate gave adverse effect on the impact wear resistance. Through impact of abrasive media on the surface, austenite in matrix was easy to transform into martensite, and it was obvious that there were much more micro-cracks in carbide and boundary of carbide and matrix. As volume of martensite increased, the abrasion occurred remarkably. This is explained by impingement heat generated which causes self-temper softening of martensite occur during abrasive blasting test.

1. Introduction

27 mass%Cr cast iron is widely used for making parts inside of shot blast machines and surface treatment machines such as blades, distributors, gauges and liners because of good thermal and high abrasive resistance. It is reported that hard eutectic carbides in a metastable austenitic/martensitic dendrite is the typical as-cast microstructure of high chromium cast irons, which is responsible for maintaining lifetime of cast parts under severe abrasive environment.1,2) Because of such as-cast microstructure, some of cast parts could be commercially used in as-cast state without heat treatment.

On the hand, iron chiller plays an important role in controlling cooling rate during solidification in order to prevent the formation of shrinkage defects in the complicated shape of cast parts. Not only growth of dendrite but also microstructure is known to be influenced in the vicinity of area directly contacting with the iron chiller.

In order to improve abrasive resistance, most of studies were undertaken by addition of alloying elements such as molybdenum, tungsten, titanium, niobium, vanadium, etc. to the high chromium cast iron or destabilization heat treatment to clarify the mutual interaction between matrix and carbide which determining abrasive resistance.37) However, there was little report on whether difference in as-cast microstructure affected to impact wear resistance of the cast iron.

In foundry, solidification rate and subsequent cooling rate are such important factors that mass production could be assured in high quality. The purpose of this study was to evaluate the influence of as-cast microstructure which cooling rate was controlled by using the iron chiller, on the impact wear resistance of 27 mass%Cr cast iron through characterization of its microstructural features.

2. Experimental Procedure

2.1 Preparation of experimental sample

A sample plate had two casting nuts for assembly, which schematic is shown in Fig. 1. For comparison, one iron chiller directly contacted the reverse surface of one casting nut for the casting sample. The iron chiller was disc shape with a diameter of 27 mm and a thickness of 20 mm. The samples were prepared in a 100 kg high-frequency-induction furnace by using raw materials, such as steel scrap, pig iron, ferroalloys and pure metals. The melt was superheated to 1823 ± 20 K. After being held for 600 seconds, the molten alloy was tapped into ladle at 1673–1723 K and then poured to the sand molds. Final chemical compositions analyzed by spectrometry were shown in Table 1.

Fig. 1

Schematic of the sample plate.

Table 1 Chemical compositions of cast iron (mass%).

Experimental samples for abrasive blasting tests and metallography were cut from cast samples by a wire cutter. The samples which the planar dimension were approximately 48 mm wide and 52 mm long, included area affected by the iron chiller.

2.2 Abrasive blasting test

In order to evaluate the wear behavior, the schematic of abrasive blasting machine used in this study is shown in Fig. 2. Iron grit (Fe–1.0C) with 850 HV and 1.2 mm in diameter was chosen as abrasive media. They were accelerated and shoot on the experimental samples by abrasive blasting under the pressure of 0.5 MPa. Impingement angle was 90°, and blast distance from the tip of 6 mm diameter nozzle to the sample surface was 100 mm. For each sample, 25 kg of iron grit impacted on the sample with 1000 seconds shooting time. During the tests, spark generation was visually observed. This process was repeated for 20 times. Before and after each 5 times test, weight loss and decrease of sample thickness were measured using analytical electronic balance and a contact profilometer, respectively.

Fig. 2

Schematic of (a) an abrasive blasting machine, and (b) blasting test; (c) Observation of spark generation during abrasive blasting.

2.3 Microstructure analysis

Experimental samples were grinded on emery papers and then buffed with alumina powder with a grain size of 5.0 and 0.5 micron. Once polished, the samples were etched with Villela’s reagent (5 ml hydrochloric acid and 1 g picric acid in 100 ml ethanol) for 7–10 s to reveal the microstructure.

Microstructure of samples before and after abrasive blasting tests was observed using a Keyence VHX-2000 optical microscope (OM) and a JEOL IT-100LA scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscope (EDS) detector. Phase constitution was measured by X-ray diffraction (XRD) in a PANalytical diffractometer using Cu-kα radiation in a 2θ range of 30–80°.

The volume of martensite was measured using with a feritscope (FMP 30). The device was calibrated with δ-ferrite samples and its measurement range was 0–80% δ-ferrite. Ten measurements were performed on each sample at arbitrary area on the sample surface.

2.4 Measurement of hardness

Hardness of samples was measured using an AVK-A type Vickers hardness tester with a load of 294.2 N (30 kgf). Ten measurements were taken for each sample. Micro hardness was determined using an HMV-G21 FA type micro Vickers hardness tester with a load of 2.94 N (0.3 kgf). It was tested from the surface toward center of sample after abrasive blasting test. Each reported value is an average of three measurements.

3. Results and Discussions

3.1 As-cast microstructure

After etching, it was visually distinguished that there were two types of area on the surface of sample attached to the chiller during solidification. As shown in Fig. 3(a), the round area observed was originally the place directly contacting with chiller. As-cast microstructure of the area was rather finer than the area unaffected by the chiller (Fig. 3(b)) and sample without it (Fig. 3(c)). XRD diffraction patterns of their microstructure were shown in same figure. Based on diffraction angle of main peaks, it was confirmed that microstructure consist of M7C3, M23C6 carbide, austenite and martensite in sample without the chiller. However, we can see that the peaks of austenite were significantly attenuated in the area where was directly contacted with the chiller. The volume of martensite measured by feritscope was 36% in the area whereas one in remain area unaffected by the chiller was about 22%, equal to sample without it. It was expected that austenite had transformed to martensite. Difference in microstructure could be explained by the solidification rate and subsequent cooling rate.

Fig. 3

Optical micrographs and XRD diffraction patterns as-cast microstructure, (a) area affected by the chiller; (b) area unaffected by the chiller; (c) sample without the chiller before the abrasive blasting.

Generally, M7C3 carbide is reported to crystallize at about 1573 K, and coarsen in the solidification process of 27 mass%Cr cast iron.8) And then, mixture microstructure of austenite, and primary carbides like M7C3, M23C6 is formed at the eutectic temperature. Nuclei generation and coarsening of the primary carbides depend on cooling rate. When the cooling rate is fast, nuclei of primary carbides easily crystallize, but coarsening of the carbides becomes difficult. In addition, formation of primary carbides reduces solute carbon content in austenitic matrix. Therefore, stability of austenite phase decreases with increment of primary carbide volume. On the other hand, the fine primary carbides suppress growth of austenite grain during cooling below the eutectic temperature. As a result, metastable austenite transformed into martensite easily when volume of fine primary carbide increased.

Hardness of the area directly affected by the chiller was 752 HV, higher than that of one unaffected by the chiller and sample without the chiller. Hardness was well consistent with the microstructure.

3.2 Impact wear behavior

The relation between weight loss, thickness reduction and number of tests were plotted in Fig. 4. In both samples, weight loss increased linearly with increment of the number of tests, but sample with the chiller did not exhibit excellent impact wear resistance than expected. Weight loss and thickness reduction of sample with the chiller were about 3 times higher than sample without the chiller. Visual appearance of surface of both samples after 20 times abrasive blasting was illustrated in Fig. 5. The cross section shown in same figure was cut along the dotted line from the central part by wire cutter. It was noticed that blasting of iron grit made a significantly large hole in the area affected by the chiller. There was almost no difference between the most worn portion and outer part of the sample without the chiller. Through cross section, it was obvious that formation of shrinkage was suppressed by the chiller, but impact wear behavior of the area affected by the chiller was most severe.

Fig. 4

Change of weight loss and decrease in thickness of the experimental samples during abrasive blasting tests.

Fig. 5

Visual appearance of surface of experimental samples and macroscopic cross sectional view of center part cutting through the dotted line after 20 times abrasive blasting.

Figure 6 is SEM micrographs of the cross section in the most worn portion of two samples at low and high magnification. In the region of 200 µm from surface, it could be observed that many micro-cracks existed at boundary of matrix and primary carbide. It was also obvious there were micro-cracks in some of carbides. As compared with sample without the chiller, amount of micro-cracks in the area affected by the chiller was much more.

Fig. 6

SEM micrographs of cross sections in the most abrasive portion of experimental samples in as-cast state after abrasive blasting test, (a) area affected by the chiller; (b) sample without the chiller.

Microhardness profile of the most worn portion of samples on the cross section is shown in Fig. 7. There was hard facing phenomenon contributed by abrasive media in both samples. However, the area affected by the chiller did not exhibit remarkable surface hardening like as sample without the chiller. Average value of microhardness near surface of sample without the chiller reached to 1050 HV, whereas it was about 800 HV of the area affected by the chiller.

Fig. 7

Change in hardness through the cross section of the most worn portion after 20 times abrasive blasting test.

Figure 8 shows the volume of martensite measured by the ferrite scope after 20 times abrasive blasting. Depressed part was the most worn portion; planar part was remaining area where was little effect from abrasive blasting. The increment of martensite was recognized in both samples after abrasive blasting. It may explained by strain-induced transformation of metastable austenite to martensite under abrasive blasting condition.9) However, increment of martensite in each sample is different. In sample without a chiller, martensite volume in the depressed part was slightly higher than that of planar part because the amount of impacted grit is larger. On the other hand, it was noticed that increment of martensite volume in depressed part where was originally the area affected by the chiller (Fig. 3(a)) was remarkable, but there is no difference in volume martensite between planar part where was the area unaffected by the chiller (Fig. 3(b)) and sample without the chiller. This could be considered by stability of austenite during solidification. As mentioned above, increment of cooling rate promotes refinement of primary carbides and austenite in as-cast state. Due to formation of fine primary carbide, reduction of solute carbon content in austenite matrix lowers the stability of austenite. Therefore, fine grains of metastable austenite promotes martensitic transformation under rapid cooling rate or by applying external force. During abrasive blasting, it was considered that spark generation which collision energy was large enough to promote self-temper softening of martensite, when iron grit impacted on surface of sample. The matrix in the vicinity of surface was easily deformed due to the increase of tempered softening martensite. As a result, deformation of matrix, and cracking in carbide and boundary of matrix and carbide accelerated abrasion more severe.

Fig. 8

Volume of martensite in experimental samples after 20 times abrasive blasting test.

The abrasion results highlight that as-cast microstructure of the matrix displays a strong influence on impact wear resistance. Austenite, which is high thermal stability, supports the carbide against cracking under abrasive blasting environment as compared with martensite.

4. Conclusion

Of the results obtained under the experimental conditions of this study, the following conclusions can be drawn.

  1. (1)    Refinement of as-cast microstructure due to increment of cooling rate during solidification significantly deteriorates the impact wear resistance of 27 mass%Cr cast iron.
  2. (2)    Under abrasive blasting condition, deformation of matrix near surface and cracking of primary carbides are more severe with increasing volume of martensite. Because martensite is softened due to self-temper softening, effect of martensite on impact wear resistance is smaller than that of austenite.

REFERENCES
 
© 2019 Japan Foundry Engineering Society
feedback
Top