Materials Transactions, Vol. 45, No. 3 (2004) pp. 880 to 887 #2004 The Japan Institute of Metals

Effect of Heat Treatment on Microstructure and Properties of Semi-solid Chromium Cast Iron Nuchthana Poolthong1 , Hiroyuki Nomura2 and Mitsuharu Takita2 1

Division of Materials Technology, School of Energy and Materials, King Mongkut’s University of Technology Thonburi, Bangkok, 10140, Thailand 2 Department of Materials Processing Engineering, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Semi-solid processed 18% Cr and 27% Cr cast irons were produced by using copper cooling plate and metal mold. A series of experiments were carried out to clarify the effect of heat treatment on microstructure, hardness, wear properties, and corrosion characteristics. The results show that 27% Cr alloys possess better abrasive resistance than 18% Cr alloys due to higher carbide volume under all the conditions when tested by dry sand rubber wheel with silica abrasive. However 18% Cr alloys show higher wear resistance than 27% Cr alloys when using super hard alloy disk against the specimen plate surface. The harder disk indent into the carbides, leads to spalling and pitting, and therefore greater wear rate. For the corrosion test result, 27% Cr alloys have better corrosion resistance than 18% Cr alloys as a result of higher chromium content. A combination of semi-solid processing and heat treatment improves wear resistance and corrosion resistance of chromium cast iron. (Received July 2, 2003; Accepted January 6, 2004) Keywords: semi-solid processing, high chromium cast iron, hardness, abrasion, wear resistance, corrosion characterization

1.

Introduction

Cooling plate

High-chromium white iron is an erosion resistant ferrous alloy widely used in manufacturing, it has for long been applied to components in mining and minerals industry due to its excellent abrasion resistance, imparted by the hard alloy eutectic carbides present in the microstructure.1) Furthermore heat treatment can improve properties, all of interest, depending upon the particular applications.2) In the present study, semi-solid processing is applied to 18% Cr and 27% Cr cast iron in order to obtain a new type cast iron with superior mechanical properties. Semi-solid processing is investigated by using semi-circular channeled copper cooling plate and characteristics of heat treatment on the microstructure, wear properties, and corrosion characteristics of 18% Cr and 27% Cr cast iron, and material properties are compared with those obtained by ordinary high chromium casting. 2.

Experimental Procedure

High chromium cast iron samples were prepared and melted in a magnesia crucible in an electric resistance furnace that can be rapidly heated up to temperature exceeding 1723 K using silicon-carbide rods. The composition of cast iron investigated is shown in Table 1. After cooling to a temperature above the liquidus, the samples were taken out of the furnace and poured under the two conditions; the ordinary sample was poured directly into a metal mold while the other prepared for semi-solid processing poured over a 500 mm-long (semi-circular channel) cooling plate made from copper and inclined at 10
, into a mold as shown in Figs. 1(a) and (b). The flow channel was coated with a thin Table 1

Chemical composition.

Sample No.

%C

%Si

%Mn

%Cr

%Ni

%Mo

%Cu

A

2.91

0.35

0.94

26.96

0.26

0.08

0.03

B

3.06

0.58

0.36

18.14

0.09

0.87

0.02

(mass %)

Ceramic funnel 10o

Metal mold Sand bed

Fig. 1 Experimental apparatus. (a) Cooling plate for metal pouring, (b) Cross section of metal mold and the position of thermocouples.

layer of boron nitride to prevent sticking of solidified iron. The semi-solid processed iron samples were subjected to various heat treatments by heating them in the furnace to the desired temperature at the rate of 473 Kh1 . Soaking period was measured from the time the sample reached the desired temperature. The treatments were performed as outlined in Table 2. The microstructure of high chromium cast iron obtained by normal casting method and semi-solid processing were investigated using samples cut off at 50 mm from the top of the specimens. An X-ray diffraction method was employed to determine the phases present in the high chromium cast iron. The peaks were measured using Cu-K
radiation (model XGT-2000W). The chemical composition of the carbides and matrix was determined using Energy Dispersive X-ray Spectrometer (EDS) in a JOEL 5800 SEM, using pure elements as standards. Vickers hardness was measured for three points in every condition; upper part outside (I), upper part inside (II) and lower part inside (III). For each indent, a load of 30 kg. was applied for 15 seconds. The measurement is carried out using samples cut off at 50 and 80 mm from the top of specimen, respectively as shown in Fig. 2. Resistance to low stress abrasion was determined by dry sand rubber wheel apparatus which conforms in most aspects to specifications of ASTM Standard G 65-94 which specifies a load of 133 N. The

Effect of Heat Treatment on Microstructure and Properties of Semi-solid Chromium Cast Iron

881

Table 2 Heat treatment condition. Sample

Heat treatment condition

A.O

Ordinary metal mould as-cast

A.S1

Semi-solid as-cast

Semi-solid processed and heat treated: A.S2

Subcritically treated (tempering) at 773 K for 2 h and air cooled

A.S3

Destabilized at 1348 K for 2 h and air cooled

A.S4

Destabilized at 1348 K for 2 h and air cooled; subcritically treated at 773 K for 2 h and air cooled

B.O

Ordinary metal mould as-cast

B.S1

Semi-solid as-cast

Semi-solid processed and heat treated: B.S2

Subcritically treated (tempering) at 773 K for 2 h and air cooled

B.S3

Destabilized at 1348 K for 2 h and air cooled

B.S4

Destabilized at 1348 K for 2 h and air cooled; subcritically treated at 773 K for 2 h and air cooled

Sample A: 27% Cr iron. B: 18% Cr iron. 10

5

For the electronic chemical experiment to determine corrosion characteristics, the potentiodynamic technique was used to analyze the anode polarization characteristics. A saturated calomel electrode (SCE) was used as a reference electrode. The anode polarization characteristics of the highchromium cast iron in 0.5 molar sulphuric acid were investigated for a voltage increase rate of 1.667 mV/s at room temperature.

50

A B

100

II I

3.

3.1 Cooling curve Figure 3 shows the cooling curves in normal casting condition for 18% Cr and 27% Cr cast iron. Solidification of

III 20

Results and Discussion

Unit: mm

abrasive used was 250–500 mm silica sand. The total test duration of each specimen was 1000 revolutions or approximately 700 m, with the mass loss of specimens being measured at every 100 revolutions. The total mass loss was plotted against the total path length and the abrasion behavior was represented by weight loss per unit path length. These measurements are done for samples cut vertically from the top of specimen as shown in Fig. 2. Friction tests were carried out using the Okoshi type wear tester which involves a loading counter and super hard alloy disk rotating against the specimen plate surface. The wear loss was determined by measuring volume change in the specimen. The wear volume from the specimen plate can be calculated from the scar dimensions. The Okoshi type wear test was carried out under a load of 6.4 kg and a friction length of 66.6 m, while the rotation velocity of the counter disk was changed within the range 0.46 to 4.36 m/s. The investigated samples were cut from the upper region of specimen as shown in Fig. 2.

Temperature, T/K

Fig. 2 Cross section of sample and the position investigated. The position I, II and III are for hardness test. The plate A and B are for low abrasion and friction test, respectively.

Time, t/s Fig. 3

Cooling curve of two types chromium cast iron.

882

N. Poolthong, H. Nomura and M. Takita

(a)

(b)

(c)

(d)

(e) 27% Cr cast iron starts at 1538 K with the formation of proeutectic austenite, followed by the monovariant eutectic reaction (L !  + M7 C3 ). The 18% Cr started its solidification at 1583 K with the formation of primary austenite and progressed through the monovariant eutectic reaction which took place over 80 K temperature interval.3) In experiments using cooling plate, molten cast iron is poured over an inclined cooling plate at room temperature. Heat is lost through the plate-wall and consequently nucleation is initialed along the meltnplate contact surface as the melt flows. Due to the melt-flow action, the nucleated crystals are drawn into the melt stream before they come into contact with adjacent crystals to form solid shell. A coating of boron nitride and graphite was applied on the surface of cooling plate to ensure that precipitating crystals separate easily from the plate wall.

10 µm

Fig. 4 Microstructures of 27% Cr alloys. (a) ordinary metal mold casting (A.O), (b) as-cast semi-solid (A.S1), (c) subcritical at 773 K for 2 hours (A.S2), (d) destabilization at 1348 K for 2 hours (A.S3), (e) destabilization at 1348 K for 2 hours and subcritical at 773 K for 2 hours (A.S4).

3.2 Microstructures and X-ray diffraction analysis Figures 4 and 5 show the microstructures of the 27% Cr and 18% Cr samples respectively, as observed under scanning electron microscopy. From the X-ray diffraction analysis results in Fig. 6, it can be concluded that the matrix of 27% Cr iron in Fig. 4(a) for ordinary metal mold casting (A.O) and in Fig. 4(b) for as-cast semi-solid sample (A.S1) are largely austenitic and martensitic. At room temperature for the as-cast semi-solid sample, the austenite partially transformed into martensite with rather appreciably high proportions of retained austenite.4) The influence of heat treatment in semi-solid process was shown in the photos of samples A.S1, A.S2, A.S3 and A.S4, where the primary proeutectic austenite became spheroidal and was dispersed uniformly in the sample when compared with specimen A.O. The subcritical heat treatment specimens, A.S2 as shown in Fig. 4(c), had essentially the same microstructure as the as-

Effect of Heat Treatment on Microstructure and Properties of Semi-solid Chromium Cast Iron

(a)

(b)

(c)

(d)

(e) cast semi-solid specimens (A.S1). At the subcritical temperature (728-793 K), no phase transformation occurs but rather very small carbides are precipitated.1) After the destabilization heat treatment at 1348 K for 2 hours, the microstructure of specimen A.S3 (Fig. 4(d)) consisted primarily of martensite (which formed during cooling), retained austenite, eutectic carbides and secondary carbides formed during the annealing. The microstructure of the destabilized and subcritically heat-treated specimen A.S4 (Fig. 4(e)) consisted of retained austenite, eutectic and secondary carbides in tempered martensitic matrix. In the as-cast condition the matrix of 18% Cr iron in Fig. 5(a) for ordinary metal mold casting (B.O) shows the presence of primary dendrites of austenite partially transformed on cooling into pearlite, bainite, and martensite. The

883

10 µm

Fig. 5 Microstructures of 18% Cr alloys. (a) ordinary metal mold casting (B.O), (b) as-cast semi-solid (B.S1), (c) subcritical at 773 K for 2 hours (B.S2), (d) destabilization at 1348 K for 2 hours (B.S3), (e) destabilization at 1348 K for 2 hours and subcritical at 773 K for 2 hours (B.S4).

matrix of as-cast semi-solid sample in Fig. 5(b) (B.S1) is composed of spheroidal primary austenite crystals which transformed to pearlite, bainite and martensite. The subcritical heat-treated specimens, B.S2 as shown in Fig. 5(c), had essentially the same microstructure as the as-cast semi-solid specimens (B.S1). After the destabilization heat treatment, the microstructure of specimen B.S3 (Fig. 5(d)) consisted primarily of martensite, retained austenite, eutectic carbides and secondary carbides. The microstructure of the destabilized and subcritically heat-treated specimen B.S4 (Fig. 5(e)) consisted of retained austenite, eutectic and secondary carbides in tempered martensitic matrix.

884

N. Poolthong, H. Nomura and M. Takita

M7C3 + BCC M7C3 + BCC

M7C3 M7C3

A.S4

M7C3

B.S4

M7C3

M7C3 + BCC M7C3 + BCC M7C3 M7C3

M7C3

B.S3

M7C3

A.S3 γ γ

A.S2

M7C3 + BCC

M7C3 + BCC

M7C3

M7C3 +γ

M7C3

B.S2

M7C3

γ

γ

M7C3 + BCC M7C3 M7C3 +γ

M7C3 +γ

B.S1

M7C3

M7C3

M7C3 + BCC

M7C3

M7C3 +γ

M7C3

A.S1 γ M7C3

M7C3

M7C3 +γ

A.O 30°

35°

40°

M7C3 + BCC

M7C3 + BCC

γ

45°

50°

M7C3

55°

60°

30°

35°

40°

M7C3

45°

50°

55°

60°

2θ

2θ Fig. 6

M7C3 +γ

B.O

X-ray diffraction spectra illustrating the phase transformation in the samples.

Energy dispersive analysis (EDS) of carbide and matrix The results of the Energy Dispersive Analysis undertaken on the carbide and matrix phases shows that large content of chromium combined with carbon in the carbides so chromium is higher in the carbide than matrix. The chromium content in the carbide of 27% Cr iron is about 37–40% and about 15–16% in the matrix. For 18% Cr iron, chromium content in the carbide is about 26–29% and in the matrix is about 9–10%. It can be explained from the point of partition coefficient of chromium. The experiment on partition coefficents of chromium and carbon in Fe-Cr-C alloys by Y. Ono et al.5) shows that the partition coefficient of chromium to carbide and hence partition ratio obtained dividing the partition coefficient to carbide by the partition coefficient to austenite is larger than unity so chromium was preferentially distributed to carbide rather that to austenite matrix. For use as corrosion resistant material, they achieve their passive nature through the formation of an invisible and adherent chromium-rich oxide surface film. The film is first observed at about 10.5% Cr, but it is rather weak at this composition and affords only mild atmospheric protection.6) So from the result of 18% Cr alloys, they did not have a protective film because the samples had chromium content in the matrix lower than 10.5%.

Table 3 Vickers hardness results.

3.3

3.4 Hardness Table 3 shows the Vickers hardness results of the specimens. In almost all the conditions, except four data, the

Sample

Upper part outside (I)

Upper part inside (II)

Lower part inside (III)

A.O

623

509

598

A.S1

655

528

639

A.S2

613

544

588

A.S3

720

683

701

A.S4

726

713

795

B.O

549

502

571

B.S1

562

558

613

B.S2

548

540

562

B.S3

820

818

863

B.S4

869

834

897

hardness of semi-solid chromium cast iron was higher than that of ordinary as-cast iron due to the effect of microstructure obtained by semi-solid processing. The results show that almost in every condition, the hardness of upper part outside was higher than the other part due to the effect of high cooling rate. The hardness of 27% Cr alloy tends to be slightly higher than the 18% Cr alloy in the case of ordinary as-cast samples, as-cast semi-solid and subcritical heattreated samples. However, for the destabilized, destabilized and subcritically heat-treated specimens, the hardness of 18% Cr alloy was slightly higher than that of 27% Cr alloy. This is related to the decrease in carbon solubility in austenite with increasing chromium and the increased removal of carbon

Effect of Heat Treatment on Microstructure and Properties of Semi-solid Chromium Cast Iron 180

160 A.S1

140

A.S2

120

Mass Loss, ∆M/mg

Mass Loss, ∆M/mg

the reason below.8,9) The carbide volume can be calculated from10)

180 A.O

160

A.S3

100 A.S4

80 60

140

%carbide volume ¼ 12:33ð%CÞ þ 0:55ð%CrÞ  15:2

120

Under the low stress abrasion conditions, several researchers have found that increasing carbide volume improves the abrasion resistance.10) High chromium cast iron commonly form the M7 C3 carbide as the eutectic carbide, which has a hardness greater than that of silica as shown in Table 4, and would therefore be expected to show excellent wear resistance against this abrasive. In wear test using silica as the abrasive, the matrix regions wore preferentially, leaving the eutectic carbide regions to support the load of the abrasive particles. By increasing the carbide volume, the proportion of support regions increased, hence improved abrasion resistance.11)

100 80 B.O

60

B.S2

40

40

B.S3 B.S4

20

20

0

0 0

400

800

885

0

1200

Sliding Distance, l/m

200

400

600

800

Sliding Distance, l/m

Fig. 7 The abrasion behavior plot of mass loss vs. sliding distance to show the effect of heat treatment.

from austenite during destabilization as secondary carbides form. The higher the chromium content, the lower the carbon content of destabilized austenite, hence, a softer, lower carbon martensite is produced on subsequent transformation.7) A maximum hardness of 897 HV was obtained for the destabilized and subcritical heat-treated specimen B.S4 as a result of very fine carbide precipitation and subsequent increase of martensitic structure. 3.5 Abrasive wear resistance The effect of heat treatment condition on the mass loss due to abrasive wear measured by the dry sand rubber wheel abrasion test was presented as a function of sliding distance in Fig. 7. The mass loss for high chromium cast iron increased linearly with increase of sliding distance. Higher abrasive loss for 27% Cr and 18% Cr alloys occurred in ascast samples (i.e., specimens A.O, A.S1 and B.O). Wear rate of the heat-treated samples were lower than as-cast ordinary and as-cast semi solid processed samples, especially for 27% Cr alloy, under all the conditions due to a microstructure consisting of martensite and retained austenite. It was found that wear rates for 18% Cr alloys were higher than for 27% Cr alloys under almost all the conditions, from

3.6 Wear by the Okoshi type test Figure 8 shows the effect of the heat treatment condition on the relationship between the specific wear resistance ratio and sliding velocity from 0.46 to 4.36 m/s. In the case of 27% Cr alloys the specific wear ratio for every sample except A.S2 increased with increase of sliding velocity until 1.64 m/s; thereafter the specific wear ratio decreased rapidly until velocity of 2.37 m/s, and then at velocities more than 2.37 m/ s, specific wear ratio had no significant change. These changes of the slope suggest that the wear mechanism change with sliding velocity range namely in slow velocity region and high velocity region. These profiles of wear curve Table 4 Hardness of abrasive materials relative to that of microstructural constituents of alloy. Abrasive materials

Vickers hardness

Silica

900–1280

Super hard alloy

1600–2500

Microstructure constituent

1200–1800

M7 C3 -type carbide

770–800

High carbon martensite

350–400

Austenite

6

6

A.S1

5

Specific Wear Ratio, ∆W/1/1013Pa

Specific Wear Ratio, ∆W/1/1013Pa

A.O A.S2 A.S3

4

A.S4 3

2

1

B.S1 B.S2

4

B.S3 B.S4

3 2 1 0

0 0

1

2

3

4

Sliding Velocity, V/ m.s-1 Fig. 8

B.O

5

5

0

1

2

3

4

5

Sliding Velocity, V/ m.s-1

The friction rate plot of specific wear ratio vs. sliding velocity for chromium cast iron to show the effect of heat treatment.

N. Poolthong, H. Nomura and M. Takita

suggest that at low sliding velocity, specimen A.S2 has the highest wear resistance but at high sliding velocity A.S4 has the best wear resistance. For the ordinary as-cast sample, it had good wear resistance at high velocity region, but at low velocity region it was not so good. At low sliding velocity, the materials containing a considerable amount of austenite in the matrix possess higher wear resistance than specimens with primarily martensitic matrix. Higher wear resistance of austenitic matrix alloys stems from several properties that this phase possesses. One of these properties is ductility. Austenite, being more ductile than martensite, plastically deforms to a greater extent without fracturing under the normal and tangential force at low sliding velocity.12) In the case of 18% Cr alloys, the specific wear ratio for every sample was slightly different under every condition and much lower than 27% Cr alloys. This is due to the result of using a harder abrasive i.e. super hard alloy disk with hardness value shown in Table 4. Increasing the chromium or carbide volume may lead to little improvement and even a decrease in the wear resistance. It has been suggested that harder abrasives indent into the carbides, leading to spalling and pitting, and therefore greater wear rates13) as shown schematically in the Fig. 9. The conditions leading to carbide fracture during abrasion are based on the concept of a ’threshold’ depth of cut with carbide. Fracture of carbides was believed to occur if the depth of cut of the abrasive particles exceeded a critical depth. Hard abrasive, high load and angular abrasive particles were thought to produce a larger depth of cut, and hence, were more likely to lead to carbide fracture.11) It is also believed the test configuration affected the depth of cut, with the test using a hard counterbody, such as carbon steel leading to a larger depth of cut.14) On the other hand, the rubber wheel test, commonly used to study low stress abrasion using a softer counterbody, may allow the abrasive particles to ride over carbide particles through deflection of the rubber wheel, reducing the depth of cut and likelihood of carbide fracture.11) Wear resistance of AO is better than AS due to carbide size. Dogan and Hawk15) found that for high stress abrasion test the volume wear rate decreases with increasing carbide volume fraction and with increasing superheat or with increasing carbide size at the same carbide volume fraction. Wear processes activated in the carbides in the high Cr white cast iron also include both plastic deformation and fracture.

Shearing is caused by the tangential forces applied by the moving abrasive particles. During bending, tensile stresses develop on the back side of the carbide rod, leading to fracture. However, as the rods become thicker, bending and fracture become more difficult under the same applied tangenial force. As for influence of microstructure on wear performance, the following summary is deduced: from the test result of low abrasive wear test and wear by the Okoshi type test, one can conclude that for abrasion with hard abrasives (super hard alloy disk), austenite matrix and low carbide volume is preferred, while a martensitic matrix and high carbide volume performs best with softer abrasives (silica). 3.7 Corrosion characteristics Anodic polarization curves obtained in comparison with corrosion behavior of different samples were presented in Fig. 10. Corrosion characteristics of the samples were listed in Table 5. These results show that the better corrosion resistance is obtained in subcritically heat-treated specimens, A.S2 and B.S2. In ordinary as-cast and semi-solid as-cast samples, the austenite regions adjacent to eutectic carbides suffer from chromium depletion. This leads to local attack of austenite around the eutectic carbides. Subcritical heat treatment improved corrosion resistance giving slightly wider passive ranges and lower values for critical and passive current densities than the as-cast condition. The more uniform chromium distribution is believed to be the cause of the improved corrosion resistance for heat-treated material.16) The 27% Cr alloys have better corrosion resistance than 18%

A.S4 A.S2

Current Density, I/ A . m-2

886

A.S3

A.O A.S1

Potential, E/mV

Super hard alloy disk

Super hard alloy disk

Brittle chip removal

Brittle chip removal

C

M

C

M

C

Current Density, I/ A . m-2

B.S4

B.O

B.S2

B.S3

C MC M C M C M C

microcracks

microcracks

Low carbides volume

High carbides volume Potential, E/mV

Fig. 9 Schematic of the relationship between super hard alloy disk and role of matrix (M) in abrasive wear of chromium cast iron: eutectic carbides designated by C.

Fig. 10 Anodic polarization curves for chromium cast iron tested in 0.5 kmol/m3 sulphuric acid.

Effect of Heat Treatment on Microstructure and Properties of Semi-solid Chromium Cast Iron Table 5

Corrosion data.

Ipass

Icorr

Passive range

Corrosion rate

(mA/cm2 )

(mA/cm2 )

(mV)

(mmpy)

A.O A.S1

0.890 1.130

2.875 2.868

560 693

29.82 29.75

A.S2

0.964

2.399

700

24.88

A.S3

1.250

4.036

648

41.86

A.S4

2.870

2.870

668

37.01

Sample

B.O

2.816

3.002

600

31.14

B.S1 B.S2

— 0.901

— 2.637

— 630

— 27.35

B.S3

2.222

7.863

540

81.56

B.S4

2.487

3.509

570

36.39

Cr alloys. The electron dispersive analysis results show that the 27% Cr alloys have chromium content more than 10.5% in their matrix so they can produce a passive film to prevent corrosion.6) In the case of 18% Cr alloys the films that form are not the complete passive film but rather a product of oxides that form during corrosion. The passivation of the iron-chromium alloys occurs in two steps as can be seen in the polarization curves. Based on the work of Gerretsen and De Wit., it is assumed that the first active-passive transition is due to the formation of a (mono-layer) chromium oxide passive film, while in the second transition iron contributes to the passivation (multi-layer film).17) From the research of Dobbelaar et al., it is found that for metals at lower concentration of alloying element, the amount of chromium is too low to cover the whole surface with chromium oxide and passivation will not be complete.18) This idea is theoretically supported by the percolation model of Sieradzki and Newman.19) A porous passive film is formed after the first peak in the polarization curve. The amount of porosity decreases with increasing chromium content. 4.

Conclusions

(1) Semi-solid processed 27% Cr cast iron structure consisted of primary spheroidal austenite particles, martensite and eutectic carbide structure. The 18% Cr semi-solid cast iron structures consisted of primary spheroidal austenite particles, pearlite, martensite and eutectic carbide structure. (2) Hardness of semi-solid chromium cast iron was higher than that of ordinary as-cast iron due to the characteristic microstructure of semi-solid processed cast iron. Hardness of 27% Cr alloys tends to be slightly higher than 18% Cr alloys in the case of as-cast and subcritically heat treated specimen but slightly lower in case of destabilized, destabilized and subcritically heat treated specimens as the result of the lower content of carbon in

(3)

(4)

(5)

(6)

887

destabilized austenite, hence, a softer, lower carbon martensite. For the results of abrasive wear test of 27% Cr and 18% Cr alloys using rubber wheel test, all heat-treated samples had the matrix containing a mixture of austenite and martensite exhibiting better wear resistance than the as-cast iron. Abrasive wear rates of 18% Cr alloys were larger than 27% Cr alloys in almost all the conditions as a result of decreased carbide volume and the fact that the as-cast samples had some pearlitic matrix as well. From the Okoshi wear test, 18% Cr alloys was shown to have the great wear resistance than 27% Cr iron when tested using a harder abrasive (super hard alloy disk). The harder disk indent into the carbides, leading to spalling and pitting, and therefore greater wear rates for higher chromium alloy with more and thinner carbides. Subcritically heat-treated samples improved corrosion resistance as a result of uniform chromium distribution. The 27% Cr iron have better corrosion resistance than 18% Cr because they have more chromium in the matrix and can produce a passive film. AS4 and AS2 are the best wear resistance for low stress abrasion and corrosion resistance, respectively, when compared with as-cast normal casting and as-cast semisolid casting.

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