The effect of strong carbide-forming elements on the adhesive wear resistance of ferritic stainless steel

The effect of strong carbide-forming elements on the adhesive wear resistance of ferritic stainless steel

Wear 249 (2001) 639–646 The effect of strong carbide-forming elements on the adhesive wear resistance of ferritic stainless steel M. Aksoy a , O. Yil...

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Wear 249 (2001) 639–646

The effect of strong carbide-forming elements on the adhesive wear resistance of ferritic stainless steel M. Aksoy a , O. Yilmaz a,∗ , M.H. Korkut b a

Department of Metallurgical Engineering, Faculty of Engineering, University of Firat, Elazi˘g, Turkey b Department of metallurgy, Faculty of Technical Education, University of Firat, Elazi˘ g, Turkey Received 10 May 2000; received in revised form 3 April 2001; accepted 10 May 2001

Abstract The influences of strong carbide-forming elements such as Mo, Ti, V, Nb and homogenization on the adhesive wear resistance of ferritic stainless steels (18 wt.% Cr) have been studied. The wear behavior of the homogenized and unhomogenized samples was investigated in a block-on-ring apparatus under the loads of 40, 60 and 80 N, respectively. The results of dry sliding tests were interpreted as taking the properties of the samples such as microstructure, surface hardness, and grain boundary energies into account. It has been found that, homogenization heat treatment decreased the weight losses. It was also observed that, except S3 for all of samples the total weight losses was changed in compatible aspects with microhardness, toughness and grain boundary energies. However, this relation could not be established between surface hardness and total weight losses. Among all of the samples, the best result has been found from the samples containing V and Mo. In addition, the samples, which consist of M23 C6 carbides in their microstructures without carbide-forming elements, gave good wear resistance under the load of 40 N. © 2001 Published by Elsevier Science B.V. Keywords: Ferritic stainless steels; Adhesive wear; Strong carbide-forming elements

1. Introduction Ferritic stainless steels do not show an allotropic property. That is why they cannot be hardened by heat treatments [1]. In their microstructures, sigma (␴)-phase and M23 C6 carbides are present. The ␴-phase, which is an intermetallic phase, increases hardness, which is sometimes useful, but also it decreases ductility, notch toughness, and corrosion resistance [1]. Thus, ␴-phase and M23 C6 carbides should be dissolved in matrix by heat treatments. This can be achieved by dissolving ␴-phase at temperatures above 900◦ C [2]. On the other hand, to avoid precipitation of M23 C6 carbides, the alloy is cooled rapidly after having been annealed at temperatures between 1050 and 1100◦ C for a time sufficient to dissolve these carbides [3]. Application of heat treatment to ferritic stainless steels increases ductility, notch toughness and corrosion resistance [4], since this treatment do not allow to the formation of ␴-phase and M23 C6 carbides again [1]. Ferritic stainless steels have gained wide acceptance in automotive exhaust systems, containers, and other functional applications owing to good fabrication at low cost, and their

∗ Corresponding author. E-mail address: [email protected] (O. Yilmaz).

0043-1648/01/$ – see front matter © 2001 Published by Elsevier Science B.V. PII: S 0 0 4 3 - 1 6 4 8 ( 0 1 ) 0 0 6 8 6 - X

resistance to chloride stress–corrosion cracking, atmospheric corrosion, and oxidation [2,3]. The effect of strong carbide-forming elements on the microstructure and abrasive wear resistance of ferritic stainless steels were investigated beforehand [5,6] and as carbide-forming elements Mo, Ti, V and Nb were used. Furthermore, these abrasive wear tests were performed over homogenized and unhomogenized samples by using SiC sand paper as abrasive. At the end of these studies, it was detected that either homogenization or the carbide-forming elements increased both wear resistance and mechanical properties. To keep on these studies, it was aimed to investigate first the amount of weight losses under different loads with dry sliding testing, second, to determine the relationships between total weight losses and mechanical properties such as notch toughness, microhardness, surface toughness and grain boundary energies.

2. Experimental procedure The chemical concentrations of the samples are given in Table 1. The alloys were cast and hot forged in the form of 20 mm diameter bars. Some of samples were subjected to homogenization at 1100◦ C for 1.5 h and water quenched.

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Table 1 The chemical compositions of the samples [5] Sample Element (wt.%)

S1 S2 S3 S4 S5

C

Mn

Si

Cr

Mo Ti

V

Nb P

S

N

0.048 0.043 0.047 0.035 0.052

0.25 0.28 0.38 0.33 0.30

0.28 0.28 0.54 0.36 0.30

18.21 18.02 18.17 18.21 17.75

– 1.0 – – –

– – – 1.0 –

– – – – 1.0

0.01 0.01 0.01 0.01 0.01

0.008 0.008 0.008 0.008 0.008

– – 1.0 – –

0.02 0.02 0.02 0.02 0.02

Homogenized and unhomogenized samples were worn under dry sliding tests by block-on-ring apparatus. The wear test specimens were prepared in the form of 12.6 mm × 12.6 mm × 10 mm. As the counter face material AISI 1050 steel ring, which was hardened to 55 Rc, was used and renewed for each test. The samples were ground in situ against 1200 grit SiC paper and then polished by diamond paste producing 0.2 ␮m rough surface and cleaned in acetone, dried and then weighed using an electronic balance having a resolution of 1 × 10−4 gr. Subsequently, samples were placed to the wear machine and the sliding wear tests were carried out in an incremental manner, i.e. 300 m per increment and 1800 m in total. After each increment, the specimen was removed, ultrasonically cleaned in acetone, weighed and then remounted in the wear tester at the same location. Three test specimens of the same test material and new steel ring were used during each test, where average were taken. The dry sliding test were carried out under the loads of 40, 60 and 80 N and the testing were applied at constant sliding velocity of 36.8 m/min in a dry air atmosphere. For metallographic investigations, samples were etched electrolytically by holding the samples in 50 ml HNO3 and 50 ml pure water solution at 1.5 V for 1.5 min. Furthermore, the surfaces were searched thoroughly by optic and SEM for microstructure determination and identification through analysis of wear tracks. Besides, the surface hardness of the samples were measured in the range of the ±3 error band with HB hardness scale under 612.5 N load. The toughness of the samples was evaluated in the range of ±0.6 error band using Charpy V-notch specimens.

3. Results and discussion 3.1. Microstructure Microstructures of the samples have been investigated in the previous papers [5,6]. Hence the microstructure results of the samples, whose dry sliding properties were investigated, are summarized here. In Fig. 1a, X-ray diffractions of unhomogenized samples are given. From these diffractions, it was seen that the microstructure of sample 1 consists of ferrite (␣-Fe), M23 C6 carbide and ␴-phase. After homogenization at 1100◦ C for

Fig. 1. (a) X-ray diffractions of unhomogenized samples; (b) X-ray diffractions of homogenized samples.

30 min, the ␴-phase is dissolved completely, but the M23 C6 carbides were not dissolved because the annealing time was not sufficient (see Fig. 1b). The ␥-austenite phase could not be seen at X-ray diffractions and microscopical investigations. Therefore, martenzite phase did not formed after applied homogenization even with 0.008% N content [5–8]. After alloying with Mo, Ti, V and Nb, MC carbides have formed in addition to (␣-Fe), M23 C6 carbide and ␴-phase as seen in X-ray diffraction given in Fig. 1a. Kuzucu et al. is given that the microstructure of S2 is very similar to S1 [6]. In Fig. 2 the microstructure of S2 before homogenization is given. As seen in Fig. 2, MoC carbides have been formed in S2. They have been formed in the form of small particles in the ferrite grains. It can be said that molybdenum addition has decreased the amount of M23 C6 carbides. That is, the ␴-phase is dissolved above 900◦ C, and M23 C6 carbides in S2 are dissolved more than S1 [6]. This is attributable to the substitution of Mo for Cr in the carbides of the Mo-bearing steel [9]. Optical and SEM investigations of unhomogenized S3 are given in Fig. 3. From this figure, it is seen that ␴-phase surrounds carbides. Eighteen Cr stainless steels at as cast

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Fig. 2. SEM micrographs of unhomogenized S2.

state [10], and at weld-sewing of these alloys, the formation of ␴-phase is expectable [2,11]. As given in Fig. 1b, and Kuzucu et al. [6] ␴-phase have dissolved, and carbides and matrix have reconstructed, because phases formed disorderly at the grain boundaries, and atoms having high energy have brought into existence [12].

Fig. 4. SEM micrograph of (a) unhomogenized and (b) homogenized S4.

The microstructure of S4 is given in Fig. 4, and SEM micrograph of the S5 is given in Fig. 5. The increase in Nb amount increased both amount of NbC carbides and reduced the amount of M23 C6 carbides [7]. It is known that Nb stabilizes the matrix and decreases M23 C6 content in austenitic and ferritic stainless steels [13]. After homogenization the M23 C6 and ␴-phase are dissolved completely. As given by Kuzucu et al. [6], the decrease in Nb content of the matrix from 0.87 to 0 wt.% Nb shows that new NbC carbides formed during homogenization. Besides Soares et al. [14] have shown that the aging process at 1100◦ C increases the amount of NbC carbides. 3.2. Wear measurements

Fig. 3. (a) Optical and (b) SEM investigations of unhomogenized S3.

Adhesive wear test results of unhomogenized samples are shown in Fig. 6a–c. The test results in Fig. 6a show that, two different characters were depicted between wear test results of the samples performed under 40 N load in which the first group was constituted from S1, S2, S4 and the second group from S3, S5. In other words, during the beginning of this

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Fig. 5. SEM micrograph of (a) unhomogenized and (b) homogenized S5.

test, the weight loss is at high amount for both S3 and S5, but going on, after 900 m sliding distance, mild wear was obtained. S1, S2 and S4 samples gave close weight loss values and from the beginning of the test, they give mild wear character. As given in Fig. 6b, increasing the load to 60 N, the weight loss of S1, S2, and S4 were increased. However, passing 600 m sliding distance, the steady state friction was observed due to mild wear. On the other hand, severe wear was detected under the load of 60 N for S3 and S5. Furthermore, by increasing the load to 80 N (see Fig. 6c) severe wear was detected at each sample due to both the leaving of the second phases (␴-phase, M23 C6 and other carbides) from their sites and participation of these carbides in wear. In Fig. 3a, it is seen that ␴-phase surrounds carbides. Thus, during friction ␴-phase brakes easily and aids to get rid of carbides from their sites. Wear test results (see Fig. 6) showed that, the least weight loss were obtained from S1, S2, S4 and the most weight loss from S3, S5 whose microstructures were represented in Figs. 2, 3b, 4a, and 5a. The cavities in grain boundaries of S5 (see Fig. 5a) are formed due to etchant causing grain

Fig. 6. Weight loss vs. sliding distance for unhomogenized samples under (a) 40 N; (b) 60 N; (c) 80 N.

boundary corrosion and resulting comes off carbides from their sites. These figures show that, the Mo and V carbides were dispersed all over the matrix in the form of fine structure and Ti, Nb carbides were formed excessively at grain boundaries in larger dimensions than Mo and V carbides. It has been reported that, the smaller the size of carbides, the higher is the wear resistance. Because, by smaller carbide size a strong bond between matrix and carbides forms, also the carbides with big dimensions cannot prevent the progress of cracks formed during dry sliding. Moreover, the fine carbides dispersed at matrix prevent these types of cracks and decreases weight loss [15,16].

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Sample S1, which contains only M23 C6 carbides, gave close weight loss result with S2 and S4 under the load of 40 and 60 N. However, under the load of 80 N, the wear behavior of this sample show severe wear behavior due to both broken off M23 C6 carbides and ␴-phase in excessive amount, and gave the most weight loss under the load of 80 N. Weight loss–sliding distance relations for homogenized samples are given in Fig. 7a–c. These curves showed that, the weight losses of homogenized samples are less than unhomogenized samples. Furthermore, after wear testing executed under the loads of 40 and 60 N (see Fig. 7a and b), S3

Fig. 8. SEM micrograph of worn surface of S3 obtained by dry sliding test performed under 80 N.

and S5 gave mild wear behavior after 600 m sliding distance. Nevertheless, S1, S2, and S4 were worn out with mild wear behavior from first to last of dry sliding tests. For this reason, the weight loss of S1, S2 and S4 gave higher values than S3 and S5’s. The dry sliding tests of S3 and S5 gave severe wear behavior under the load of 80 N (see Fig. 7c). However, it was seen that, the least weight loss was obtained for S2 and S4, which denoted mild wear from the beginning of wear to the end. On the other hand, S1 show mild wear from the beginning of wear to the end, but it gave higher weight loss values than S2 and S4. Since, like S2 and S4 samples, it does not have carbides in its microstructure. In Fig. 8, the worn surface of S3 obtained under the load 80 N is represented. Drawing a conclusion, under the load 80 N, the least weight loss was obtained for S2 and S4 which denoted mild wear from the beginning of wear to the end. As an example to worn surfaces of these samples, the surface of S2 is given in Fig. 9. Total weight losses of homogenized and unhomogenized samples were given in Table 2. The comparison between

Fig. 7. Weight loss vs. sliding distance for homogenized samples under (a) 40 N; (b) 60 N; (c) 80 N.

Fig. 9. SEM micrograph of worn surface of S2 obtained by dry sliding test performed under 80 N.

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Table 2 Total weight loss of homogenized and unhomogenized samples Sample

Total weight loss (×10−4 gr) 40 N

S1 S2 S3 S4 S5

60 N

80 N

Homogenized

Unhomogenized

Homogenized

Unhomogenized

Homogenized

Unhomogenized

54 61 122 48 84

81 75 218 68 132

103 85 153 77 229

305 270 2180 250 2900

530 123 3011 98 3445

3790 2300 4190 2000 3885

these values shows that, the total weight losses of homogenized samples are less than unhomogenized samples and the best result were taken from S2 and S4. As indicated, the total weight losses were decreased by homogenization. The decrease in the total weight loss of S1 is connected with the dissolution of the ␴-phase [5]. At the same time, the decrease in the total weight loss of S2, S3 and S5 originates from both dissolution of the ␴-phase and aging of Mo, Ti and Nb, and the decrease in the total weight loss of S4 is due to dimensional shrinking of VC carbides [5]. The oxides detected on the surface of the samples worn with severe and mild behavior are represented in Fig. 10a and b. From these figures, it was observed that, the type of oxide and the bond between the oxides and the surface affected the wear behavior of the surface as differently. The oxides appeared on the surface of the samples giving mild and severe wear behavior determines the character of the wear [17–20]. In ferrous materials, Fe2 O3 forms at low temperatures and decreases weight losses. However, at higher temperatures, FeO forms and increases weight loss [21–25]. The temperature raised during friction depends on both the plastic deformation and break off particles from the surface. The conditions causing the break off particles

from the surface also affects and increases the temperature formed from the friction between contacting surfaces [21–25]. By this increase in temperature, at worn surface, FeO, which is a brittle oxide type, have been formed and it increased weight loss by being removed from the surface [17,18]. As well, the Cr2 O3 oxide, which is a type of oxide arising on the surface of the steels containing high amount of Cr [26–29], were detected at the samples which gave high amount of weight loss. This oxide forms slowly and the crystals with sub-micron dimensions form a glaze surface of FeCrO4 by agglomeration during friction at oxide debris [30]. The carbides strengthening the matrix supply fewer breaks off particles from the worn surface. For this reason the temperature of the surface does not increases. Hence, the oxide of Fe3 O4 formed on the surface of the S2 and S4, and these samples have fine dispersed carbides, which have strengthened the matrix. The microstructure of S4 and S5 are indicated in Figs. 4b and 5b as example for comparison between the dimension of these carbides in the samples, which gave the best and the worst weight loss values. The relationship between total weight loss obtained by dry sliding testing and microhardness, notch toughness and grain boundary energies were represented in Fig. 11a–c. Upon these results a compatible relationship between

Fig. 10. X-ray diffractions of the oxides detected on the surface of samples worn by (a) mild; (b) severe wear.

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4. Conclusions 1. The least weight loss of unhomogenized samples was obtained from S2 and S4. In addition, the distinction between the weights loses of the first group (S1, S2, S4) and the second group (S3 and S5 samples) was increased by the increase in amount of load. Furthermore, for S3 and S5, the increase of loading caused to the transition from mild wear to severe wear behavior under the load of 60 N, and this transition was observed for all of the samples at the load of 80 N. The reason for the least weight loss result of S2 and S4 can be concluded from the affirmative effect of structures, dispersion and dimensions of V and Mo carbides. In addition, the type of the oxides formed on the surface of the samples also became affective by protecting the surfaces and decreasing weight losses. 2. Homogenization operation decreased the weight loss values of all samples because of both the decomposition of the ␴-phase and M23 C6 carbides in a great amount. After homogenization heat treatment, S1, S2 and S4 samples have been worn under mild wear behavior at all loads, but S3 and S5 have shown mild wear behavior under the loads 40 and 60 N and severe wear behavior under the load 80 N. 3. We have observed a relationship between weight loss and the type of oxides formed over the worn surface. The surface protecting oxides were detected on the mild worn surface and the brittle oxides were detected on the severe worn surface. 4. In homogenized samples, a compatible relationship between total weight loss, toughness and microhardness were detected. However, these relations could not be seen between surface hardness and total weight loss values. On the other hand, except for S3 between grain boundary energies and total weight loses a compatible change has been seen, with increase in grain boundary energies total weight loses were increased. References

Fig. 11. The variation of the total weight loss vs. (a) microhardness; (b) toughness; (c) grain boundary energy of homogenized samples under 40 N load.

microhardness, toughness and total weight loss were established. However, between weight loss and surface hardness, any compatible relation could not be detected that is why; it is not given. In Fig. 11, it was observed that, weight loss was increased with both decrease in toughness and increase in microhardness. Furthermore, except S3 for other samples, a compatible relationship between grain boundary energies and total weight loss was observed. It has also been confirmed by Jha et al. [31,32] that total weight loss decreases with decreasing grain boundary energies.

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