2013 Volume 53 Issue 5 Pages 887-893
The purpose of this study was to examine the effect of boronising heat treatment on the corrosion behaviour (in two different corrosive media) and wear properties (on two different counter sliding discs) of the DIN 1.4777 quality cast steel containing 1.7% C, 30% Cr and 1.1% Si. The steel supplied as cast was exposed to homogenisation heat treatment at 1150°C for 3,5 hours and then to boronising heat treatment at 900°C for 8 hours using the powder of Ekabor 2. An optical light microscope, SEM and XRD analyses were used to conduct microstructural characterisation of the steel investigated. Electrochemical potentiodynamic polarisation measurements were taken to evaluate corrosion behaviours of the examined steels. Wear tests were conducted in a pin-on-disc type wear device by using a load between 10 N and 60 N. While the corrosion resistance of the boronising heat treated steel deteriorated by the pitting damage mechanism within corrosive media, uniform corrosion damage enhanced the corrosion resistance of the examined boronised steel. Oxidative adhesion, cracking of oxide and/or boride layer, and severe plastic deformation mechanisms were dominant during the wear tests. Severe plastic deformation and cracking of the oxide and/or boride layer caused wear mechanism to transform from mild to severe.
High chromium cast steels constitute the most important material class, in terms of superior corrosion and high temperature properties, as well as wear resistance. These steels, known as heat resisting alloy steels, are designed to resist against oxidation and reaction of gases at high temperatures. Therefore, some examples for the commercial uses of these steels, in which 30% chromium and 1% silicone are added together, are gas turbines, military equipments, chemical process equipments and aircraft engines.1,2,3,4,5,6) Throughout the country these steels are generally used in cement plants and combustion furnaces of clay pits.
The microstructure of these steels is mainly composed of a matrix, carbides and intermetallics formed after casting. Depending on the composition, the matrix can be austenitic, ferritic or martensitic. Chromium, which is a strong carbide forming element, stabilises ferritic matrix. However, Ni stabilises austenitic matrix and decreases carbide precipitation. Moreover, sigma phase is the main type of intermetallics appearing in the microstructures.7) During the casting process or subsequent heat treatment at different temperature ranges, carbides may form easily in the alloys. It is known that formation of coarse Cr23C6 in the microstructure of stainless and/or cast steels reduces the amount of free Cr dissolved in the matrix leading a decrease in the corrosion resistance.8) Generally, carbide particles in these alloys are harder than matrix and act as hard particles under dry wear condition. Such hard particles are helpful to reduce the wear rate. However, the carbide particles in these alloys commonly act as cathodic phases in corrosive media and are classified into two groups; the strong cathodic phase having low corrosion potential, such as Fe3C, Cr23C6, etc, and the weak cathodic phase that even can be oxidised in acidic media, such as intermetallic compound phase sigma.9,10,11,12,13,14,15,16,17,18)
There are limited number of studies19,20) on wear and corrosion behaviour of steels containing high amount of Cr (i.e. more than 25% Cr) and a boronising surface. Previous studies in the literature mostly focused on stainless steels and/or low alloy Cr–Ni steels.20,21,22,23,24,25,26,27) This study investigated the effect of the boronising heat treatment on the corrosion resistance of cast steels containing a low content of Ni (0.5%), 30% Cr, 1.7% C and 1% Si in two different corrosive media and their wear behaviour on two different counter sliding steel discs having different hardness.28)
In this study, after the DIN1.4777 quality steel, acquired as cast and whose chemical composition is illustrated in Table 1, was homogenised at 1150°C for 3.5 hours and some of the steels were subjected to the boronising heat treatment following the homogenisation heat treatment. The boronising heat treatment was carried out by placing the investigated steels in the EKABOR 2 powder, which was used as a boron source, in a closed stainless steel cup with 120 mm diameter and 50 mm height, in a way to hold the steels a distance of 12 mm from the edges of the cup. The boronising heat treatment was performed at 900°C for 8 hours.
The examined steels, that were homogenised and subjected to boronising heat treatment, were exposed to characterisation and mechanical tests. Characterisation of the steels was performed using metallographic examination, EDS-equipped SEM, X-rays analysis and hardness measurements. For the microstructural characterisation of the steels, they were polished using standard methods, etched as electrolytic in the etching solution (oxalic acid) under 1.5 volts for 15 seconds and then examined using a Leica DM ILM model optical light microscope and a ZEISS LS10 model SEM equipped with EDS. Philips PW1710 model XRD device was used to conduct structural analyses of the steels, which were both homogenised and exposed to the boronising heat treatment. XRD studies were performed using the Cu–Kα radiation at the current of 40 mA under the generator voltage of 40 kV in the angle range of 10°–90° and with a scan rate of 0.02 0/s. Hardness of the cross section of all examined steels, which were prepared metallographically, was measured using Vickers indenter on a Schimadzu HMV2 model micro-hardness device under a load of 2000 g. Average of at least 10 measurements was taken to determine hardness values.
A computer-controlled Gamry model PC4/300 mA potentiostat/galvanostat device with DC105 Corrosion Analysis software was used to carry out potentiodynamic corrosion tests. Corrosion test samples, which were soldered with a 1.5 mm diameter and 150 mm long copper wire on the rear surface to enable the conductivity and moulded with resin in a way to leave out only their surfaces that were required to be in contact with electrolyte were prepared using the metallographic method. Two different solutions containing 10% H2SO4 and 10% NaCl were used as the corrosive media in the experimental cell. The experiments were performed at room temperature. The steels having a stable surface area of 0.785 cm2 as the working electrode, a carbon electrode with 6 mm diameter acting as counter electrode and the saturated calomel electrode (SCE) as reference electrode were placed within the cell. While the surfaces of the working electrode and carbon electrode were mutually facing placed at a certain distance, the reference electrode was placed as closer to the working electrode in the cell.
During the corrosion tests, after the working and reference electrodes were immersed within the electrolyte, the change in corrosion potentials in terms of mV between these two electrodes was measured against time for the first 45 minutes. Reaching the equilibrium potential, potentiodynamic polarisation curves were drawn by scanning the potential from –1.00 V to 1.00 V in the scan range of 1 mV/s, from cathodic direction to anodic direction.
Wear tests were conducted using the examined steels with 30-mm height and 4-mm diameter on the counter sliding discs manufactured from the DIN 1.7225 (AISI 4140) quality tempered steel (198 HB) and DIN 1.2379 quality cold work tool steel (62 HRC) under normal atmospheric conditions in the pin-on-disc device complying with ASTM G99 standard. Four different loads between 10–60 N were applied to the wear test samples. The wear tests were carried out with a sliding rate of 0.2 m/s and at a sliding distance of 2000 m and 4000 m. The computer-controlled load cell, which was installed to the device, was used to determine friction coefficient during the wearing. The wear amount was identified in accordance with principles of weight loss measurement. An electronic scale with precision of 0.1 mg was used to determine the weight losses appearing as a result of wear experiments.
The corroded and/or worn surfaces were investigated using EDS-equipped SEM devices in order to identify both the corrosion and the wear mechanisms.
Figure 1 illustrates the SEM images of the microstructure of the steels, which was homogenised and boronised within the scope of this study. Figure 2 illustrates the XRD analysis that was taken from the steel homogenised and then boronised. The microstructure includes the matrix (white-coloured area) and carbides (semi-needle and semi-spherical islets). It is observed that a dark coloured layer, which separated from the matrix with a straight border, formed on the surface of the steel that was subjected to the boronising heat treatment. The thickness of this layer is approximately 100 μm.
Scanning Electron Microscopy images of the (a) homogenised and (b) boronised DIN 1.4777 quality steels.
XRD patterns of the homogenised and then boronised steel.
The XRD analysis of the investigated steel surfaces (Fig. 2), which was homogenised and then subjected to the boronising heat treatment, found out that the matrix was austenite and the carbide formed on the structure was Cr23C6. After the boronising heat treatment applied to the homogenised steel, a layer containing the phase mixture of CrB and Fe2B formed on the surface. The steel containing a high amount of carbon and Cr examined in this study remained austenitic at room temperature in accord with both the equivalance rate (Creq/Nieq) and composition (Modified Schaeffler and WRC 1992) diagrams.16,17,28) M23C6 type carbides were determined as the primary carbide phase on the high Cr–Ni cast steels; where the M represents the sum of Fe together with carbide forming elements that added into the high Cr–Ni cast steels.29,30,31)
Fadilha7) and Sun16,17) reported that the matrixes having compositions of “55% Fe, 29% Cr, 11% Mo and 5% Ni” and “27.5% Cr, 1.2% Si, 0.33% C and 4.65% Ni” were ferritic and carbides were Cr rich M23C6 type carbides. Even though Cr content of the investigated DIN 1.4777 quality steel was high the matrix remained austenitic due to high carbon and low Ni content were also supported by studies in the literature.15,32,33)
Since the steel has a high content of carbon and Cr, the columnar (protrusion-intended and/or saw-toothed) structure, which was expected to form on the interface of the coating/matrix after the boronising heat treatment, did not comprise.34,35,36) The columnar structure occurring on the interface of the coating/matrix reinforces the bonding of boride layer to base metal.36) Additionally, the excess carbon and Cr content caused to reduce the layer thickness due to aggravating diffusion of the boron atoms.32,33,34,35) While the morphology of the layer-low carbon borided steel substrate interfaces is saw-toothed, structural examinations of the surface of high alloy borided steels show the being of layer with flat-front morphology at the growth interfaces.37) The XRD patterns revealed that the layer containing the phase mixture of CrB and Fe2B formed on the surface of the boronised steel and this result was supported by studies available in the literature.36,38)
The hardness of the cross section of the homogenised and boronised steel was 275 HV and 2160 HV, respectively. The hardness of the boride layer was approximately 8 times higher in comparison to that of the matrix of the homogenised steel. Increase in the hardness was atributed to the formation of the boride layer with CrB and Fe2B phases.
3.2. Corrosion ResultsFigure 3 illustrates potentiodynamic polarisation curves obtained from the corrosion tests of the steels, which were conducted with 10% H2SO4 and 10% NaCI solutions. Table 2 lists the corrosion current density (Icor) and corrosion potential (Ecor) values calculated from these curves by DC 105 corrosion analysis software.
The potentiodynamic polarisation curves of the both homogenised and boronised steels in (a) 10% NaCl and (b) 10% H2SO4 solutions.
| Corrosive Media | Homogenised steel | Boronised steel | ||
|---|---|---|---|---|
| Icor (mA/cm2) | Ecor (mV) | Icor (mA/cm2) | Ecor (mV) | |
| 10% NaCl | 0.00143 | –498 | 0.00279 | –543 |
| 10% H2SO4 | 20.5 | –466 | 1.986 | –423 |
| Applied Loads (N) | Homogenised steel | Boronised steel | |
|---|---|---|---|
| DIN 1.7225 Counter Sliding Disc | DIN 1.2379 Counter Sliding Disc | DIN 1.2379 Counter Sliding Disc | |
| 10 | 0.38 | 0.69 | 0.30 |
| 20 | 0.35 | 0.23 | 0.48 |
| 40 | 0.34 | 0.21 | 0.44 |
| 60 | 0.37 | 0.09 | 0.53 |
While the both homogenised and boronised steels showed a tendency of passivation in 10% H2SO4 solution (Fig. 3), passivation did not occur in 10% NaCl solution. Furthermore, the corrosion rate of the homogenised and/or then boronised steel in the more aggressive 10% H2SO4 solution was higher than the corrosion rate in 10% NaCl solution. In Comparison to the homogenised steel, while the corrosion resistance of the steel subjected to the boronising heat treatment increased in 10% H2SO4 solution, it decreased in 10% NaCl solution. The corrosion current density decreased approximately 10 times in 10% H2SO4 solution and increased 2 times in 10% NaCl solution by applying boronising heat treatment to the homogenised steels. The change in the corrosion potential, which developed in compliance with the results of the corrosion current density, was slight (approximately 10%).
The corrosion damage of the homogenised steel in the solution of 10% H2SO4 (Fig. 4) was an uniform corrosion such as staining of the matrix and carbides after etching of the steel with etchant solution. Moreover, a localised corrosion occurred as pitting on the surface in the solution of 10% NaCI (Fig. 4). The corrosion images of the boronised steel in 10% H2SO4 solution (Fig. 4) was an uniform corrosion, as similar to that of the homogenised steel; but difference was that the uniform corrosion of the boronised steel was not as etching of the matrix. The corrosion tests of the boronised steel performed in 10% NaCI solution depicted that the boride layer locally flaked from the surface (Fig. 4). The fact that the corrosion of the boronised steel in 10%NaCl solution developed by locally flaking of the boride layer from the surface caused both an increase in the corrosion current density and a bit decrease in the corrosion potential. The presence of the corrosion damage as flaking of the boride layer was the pitting mechanism. It is thought that a galvanic couple formed on the interface when the pits occurring16,19,20,21) on the surface reached to the coating interface and the corrosion evolved with flaking of the coating having a straight interface with the matrix, as stated in Section 3.2.
SEM images of the homogenised and boronised steel corroded in 10% NaCl and 10% H2SO4 solutions.
In this study, Fig. 5 illustrates the weight loss of the investigated steels on two different-quality counter sliding discs under different loads with sliding distance. As illustrated in Fig. 5, the weight loss increased linearly with increasing the sliding distance under a certain load. The change in weight loss of the steels with sliding distance varied depending on the applied load and hardness of the counter sliding discs. While the change in the weight loss was inconsiderable under low loads (Fig. 5), the weight loss increased at high loads in a given sliding distance. The increase in the weight loss with the applied load became evident when using the harder counter sliding disc.
The change in the weight loss of the examined steels with sliding distance on the (a) DIN 1.7225 and (b) DIN 1.2379 quality counter sliding discs.
The slope of “weight loss- sliding distance” graphics, illustrated in Fig. 5, which is called as wear rate in g/m was given in Fig. 6 in accordance with the applied load. Except for the homogenised steel that was worn on the DIN1.7225 quality counter sliding disc, the homogenised and boronised steels that were tested on the DIN1.2379 quality counter sliding disc displayed a transition from mild wear to severe wear with the applied load. The homogenised steel that was worn on the DIN1.7225 quality counter sliding disc had a mild wear mechanism under all of the applied loads. On the DIN1.2379 quality counter sliding disc, while a mild wear occurred at loads lower than 20 N, the wear became severe with increasing loads above 20 N. The present results are congruent to the previous studies9,10,11,12,13,14,15,16,17,18) that wear resistance improves related to the hardness of the sliding materials, but gets worse inversly to that of the counter sliding disc.
The change in the wear rate of the examined steels with the applied loads.
SEM images and EDS analysis of the worn surfaces of the homogenised steel on two different-quality counter sliding discs under 10 N and 60 N loads are given in Fig. 7 and Table 4, respectively. The wear mechanism of the homogenised steels was adhesive and their worn surfaces were smooth and coated with an oxide film. Wear protective films, such as oxide films, break down metal to metal contact occurs. The degree of wear is qualitatively described as mild or severe. Mild wear is restricted to the outer surface layers, with the surface remaining relatively smooth and protected by the oxide layer. Besides, together with the increase in the hardness of the counter sliding disc under low loads, the roughness of the worn surface increased slightly.
SEM images of worn surfaces of the homogenised steels on the DIN1.7225 and DIN1.2379 quality counter sliding discs at 10 and 60 N loads.
| Elements wt.% | DIN 1.7225 Counter Sliding Disc | DIN 1.2379 Counter Sliding Disc | ||
|---|---|---|---|---|
| 10 N | 60 N | 10 N | 60 N | |
| Fe | 57.96 | 64.22 | 56.23 | 64.22 |
| Cr | 12.67 | 7.84 | 16.08 | 7.84 |
| Si | 12.67 | 0.56 | 0.72 | 0.56 |
| C | 5.02 | 6.81 | 7.78 | 4.81 |
| O | 23.76 | 22.57 | 19.19 | 22.57 |
Severe plastic deformation together with adhesive and oxidative wear mechanisms formed under high loads. The severe plastic deformation caused two types wear mechanism; the fracture and separation of the oxide film on the counter sliding disc with low hardness and the crushing and thinning of the oxide film on the counter sliding disc with high hardness. Under strenuous conditions, severe wear arises when the oxide layer could be ineffective, with the contact becoming metallic causing the surface to become highly deformed and wear significantly increases.
Figure 8 illustrates SEM image of worn surface of the boronised steel on the harder counter sliding disc under the load of 10 N and 60 N, respectively. In comparison to the homogenised steel on the same counter sliding disc (Fig. 7), the wearing of the boronised steel developed as the cracking and flaking of the boride layer, resulting in the contact becoming metallic. “The percentage of flaking boride layer” increased with increasing the applied load as well. The wear loss, which occurred as the flaking of the boride layer during the wear test, could be attributed to the formation of the straight border interface between the boride layer and the matrix. Compared to peel off the protective oxide layer (Fig. 7), the flaking of the boride layer (Fig. 8) resulted in a further increase in the wear rate (Fig. 6), as reported in Ref. 39). However further increase in the wear rate and flaking of the boride layer could be derived from brittle CrB and Fe2B phases and the straight interface of the coating matrix instead of saw-toothed interface of that.
SEM images and EDS results of worn surfaces of the boronised steels on the DIN1.2379 quality counter sliding disc at (a) 10 N and (b) 60 N loads.
Table 3 lists the change in the friction coefficient depending on the applied load. While the friction coefficient of the homogenised steel was almost constant under all applied loads on the DIN 1.7225 quality counter sliding disc having low hardness, it decreased on the DIN 1.2379 quality counter sliding disc having high hardness. At 10 N load the friction coefficient of the homogenised steel was the highest on the harder counter sliding disc. The crackage of the oxide film under high loads (60 N) on the counter sliding disc with low hardness did not change the friction coefficient. On the counter sliding disc with high hardness and at high (60 N) load the oxide film was not ruptured (Fig. 7) and extremely low friction was maintained throughout the test. An interesting finding of the few years has indicated that the friction coefficient decreases for the range of material combination (Ag, Au, Pl and Ru) with increasing load, as seen in this study. This behaviour has been attributed to the transition from elastic to plastic deformation.40) As illustrated in Fig. 7, the wear mechanism of the homogenised steel on the DIN 1.2379 quality counter sliding disc was the severe plastic deformation.
The friction coefficient of the boronised steel on the DIN 1.2379 quality counter sliding disc increased with the increase in the applied load. While the friction coefficient of the boronised steel was the lowest under the load of 10 N, it was the highest under the load of 60 N. The high friction coefficient under high loads could be associated with the fact that the boride layer with high hardness (Fig. 8) ruptured and slided into the friction interface during the wearing.
The following conclusions can be drawn from the results of the present investigation conducted on the corrosion and wear of the homogenised and boronised high carbon and chromium steels.
(1) The investigated homogenised steel having 32.2 of Cr equivalence and 52.5 Ni equivalence was austenitic and carbides were Cr rich M23C6 type carbides. Boride layer on the surface of the homogenised steel was approximately 100 μm in tickness and contained the phase mixture of CrB and Fe2B. The hardness of the homogenised steel increased from 275 HV to 2160 HV with applied boronising heat treatment following homogenisation heat treatment.
(2) Corrosion tests of the both homogenised and boronised steels revealed that 10% H2SO4 solution was higher aggrassive than 10% NaCl solution. Corrosion resistance of the investigated steels in 10% H2SO4 solution increased by boronising heat treatment. In addition that corrosion resistance in 10% NaCl solution exacerbated by applying boronising heat treatment to the homogenised steel. Localised corrosion morphology in 10% NaCl solution changed as an uniform demage by using 10% H2SO4 solution.
(3) Wear of the homogenised steel depending on sliding distance was mild under all applied loads on the counter sliding disc with low hardness and severe under high loads on the counter sliding disc with high hardness. The wear behaviour of the boronised steel tested on the harder counter sliding disc was similar with that of the homogenised steel, worn on the same disc, and the boronised steel displayed a greater wear in comparison to the homogenised steel. Wear rate of the investigated steels on the harder counter sliding disc was almost maintained constant up to a critical load of 20 N above which wear resistance decreased dramatically with increasing the applied loads.
This work was supported by Karabuk University Project of Researchment and Development (Code: KBU-BAP-C-11.Y011).
