Steels are commonly used in high-performance demanding applications due to their favourable mechanical properties. Various surface engineering techniques have been developed for steels, among which chromizing is an affordable high-throughput case-hardenig process for improved surface hardness and wear resistance while retaining the substrate ductility and toughness. In this work, tribological testing along with nano- and micro-indentation and morphological and composional characterisation were used to understand the effects of the chromizing process on the AISI 1095 carbon steel, 52100 bearing steel and A2, D2 and M2 tool steels. The results of this study demonstrate that the chromizing treatment of low-cost 1095 and 52100 steels significantly improves their wear and hardness properties to a level comparable to the more costly tool steels. While chromizing also increased the hardness of the tool steels, it had little improvement on the wear resistance for the D2 and M2 tool steels.
Introduction
Steels possess high hardness, toughness, wear resistance and durability [1]. Due to their favourable mechanical properties, steel materials are widely used in a variety of engineering applications with sliding interfaces such as cutting tool materials in metal cutting, wood machining [1–3], biomass comminution systems [4–6], machinery components in bearings [7], gears [8] and shafts [9] or as rails and wheel materials in the railways sector [10].
Since these applications are industry-driven, performance and cost play an important role in the materials selection [1]. Although high-alloy steels such as tool steels have higher hardness and wear resistance than low-alloy carbon steels, their cost is much higher. More affordable carbon steels could potentially achieve similar mechanical properties to those of tools steels by applying a suitable surface treatment.
Among different coating and diffusion-based surface treatment techniques, chromizing has been applied to ferrous alloys to increase their corrosion and high-temperature oxidation resistance [11–14] and to improve the wear [12–15], hardness [16–18], and adhesion properties [15,17]. The main advantage of chromizing over other surface hardening methods such as boron or titanium treatments is corrosion resistance [9]. Due to these favourable properties, chromized steel materials are used in applications such as heat exchanger tubing [19,20], timing chains in vehicles [19], automotive exhausts [21] or sealing cans for the food industry [19]. Another potential application of chromizing is the tooling components for biomass preprocessing, such as cutters and blades for reducing the size of biomass by shearing and grinding that could experience excessive wear and damage [4–6,22–25]. In the past, our efforts focused on evaluating various coatings and surface treatment methods such as diamond-like carbon, iron boriding, nickel boriding or tungsten carbide to improve the wear resistance of the cutting tools for biomass pre-processing [6]. In this work, we investigate chromizing as another potential surface treatment method.
Chromizing can be typically accomplished by different methods such as powder pack cementation [15,26,27], molten salt bath [28], fluidized beds [29], vacuum [30] or chemical [31] and physical vapour deposition [32] (CVD, PVD) techniques. Among these different methods, pack-chromizing is a more affordable, simpler and higher throughput solution for chromizing steel surfaces [26,27,33]. This process typically requires temperatures above 1000°C for a couple of hours [19,20,26,33]. The surface chromizing is a single-cycle diffusion process [14,19]. First, Cr is inwardly diffused into the steel matrix that forms the ferritic phase followed by carbon outward diffusion from the steel matrix. Due to a high affinity of Cr for carbon, Cr-carbides form at the interface while a layer beneath the interface becomes decarburized. It is known that an increasing content of carbon in the steel matrix decreases the thickness of the chromized layer [14,17,19,20]. Formation of Cr-carbides during the chromizing process lowers the diffusion rate of Cr into the underlying substrate. Alloying compounds can either increase or decrease the thickness of the chromized layer. Ni, Mn, Co, S, and P tend to produce thinner chromized layers while Cr, Si, Al, V, Mo and W are more likely to promote thicker layers [19]. However, the thickness of the layer in high-alloy steels depends on the combination and concentration of different alloying elements.
Literature lacks comprehensive tribological characterization of chromized steel materials with various compositions. Several studies attempted to characterize the tribological properties of chromized steels using a scratch test. A higher chromizing temperature and time were shown to increase the critical loads of Cr-1095 steels (chromized steel hereinafter referred to simply as Cr-) due to the formation of a thicker chromized layer [15]. Chromized steels with higher carbon content showed a higher hardness but a lower scratch resistance, which was attributed to increased delamination and cracking of the chromized layer [17]. Tribological characterization in a ball-on-flat non-lubricated configuration of chromized GB GCr15 steel (equivalent to AISI 52100 steel) showed a formation of chromium oxide tribolayer that increased the wear resistance and lowered the friction coefficient at increased contact pressures [34].
Nevertheless, the effect of diffusion chromizing on the abrasive wear resistance of steels with different alloying content has not been systematically investigated. In this work, thermal pack-chromizing was carried out on several steel materials including AISI 1095 carbon steel, 52100 bearing alloy steel, and A2, D2 and M2 tool steels. The tribological and mechanical properties were probed via two-body abrasive wear testing, microindentation, nanoindentation and microstructural characterization. The experimental and morphological data are analysed and used to explain the role of a chromized surface layer on the wear and hardness properties of steel materials with different alloy compositions.
Experimental methods
Materials and Sample Preparation
Commercial tool steels A2 (X100CrMoV5), M2 (W6Mo5Cr4V2) and D2 (X153CrMo12) (all with a nominal hardness of HRC 61), AISI 1095 carbon steel (HRC 50) and 52100 (GCr15) bearing steel (HRC 61) were selected for chromium diffusion, Table 1. All steel materials were approximately 8 mm wide, 30 mm long with various thicknesses ranging from 1 to 3 mm. Prior to chromizing, the steel samples were first polished with silicon carbide paper grit 240 and finished with grit 400.
Table 1 Elemental composition of steel substrates in wt%.
C
Mn
Si
Cr
W
Mo
V
1095
0.95
0.4
-
-
-
-
-
52100
1.0
0.3
0.23
1.4
-
-
-
A2
1.0
0.9
0.3
5.2
-
1.1
0.2
D2
1.5
0.5
0.4
12
-
0.8
0.9
M2
0.9
0.3
0.3
4.1
6.4
5
1.9
The pack chromizing of the steel materials was carried out by Applied Thermal Coatings, Inc (ATC, Chattanooga, TN). The samples were placed in a furnace and contained within a sealed retort that included a chromium-rich powder. The chromizing process was carried out at a temperature of 980°C for 8 h in an argon atmosphere. Chromized steel samples were then allowed to cool in the furnace after the diffusion process. The specimens were cross-sectioned using a slow-speed diamond saw, embedded in epoxy, and polished for microstructural and compositional characterization as well as nanoindentation.
Tribological Testing
Two-body abrasive wear resistance of the chromized steel materials was evaluated using the ASTM G174 Standard Test Method for Measuring Abrasion Resistance of Materials by Abrasive Loop Contact [35]. In this apparatus, a test specimen is placed on an adjustable arm that is loaded with a 200-gram loading mass sliding against a 30-μm Al2O3 finishing tape which was attached to a spindle that provides a unidirectional motion, Figure 1a,b. The abrasive wear experiments were performed for a total of 680 unidirectional sliding cycles, equivalent of ∼898 m of sliding at a speed of 0.25 m/s. The specimen was in contact with the unworn tape only for the initial sliding cycle and the tape continuously wore down as the sliding progressed. A new tape was used in each test. After completion of the wear testing, the worn surface topography of the samples was determined with a 3D white-light optical profiler (Wyko NT9100), Figure 1c. The worn volume Vworn [mm3] was determined using a procedure described by Budinski [36] by measuring the contact width b [mm] and contact length t [mm] as:
(1)
where D [mm] is the diameter of the spindle. The contact width b was measured at 3 different locations along the wear track and the final b was determined as a mean of the three measurements, Figure 1d. The final wear rate K [mm3/(N-m)] was then determined as Vworn divided by the applied load Fn [N] times the sliding distance d [m] as:
(2)
Figure 1 Wear testing and characterization procedure. (a) Schematic of the ASTM G174 loop abrasion wear test setup. (b) Example optical image of wear scar after 680 sliding cycles. (c) Example worn surface topography. (d) Example wear scar 2D profiles at 3 different locations from c).
Microindentation
Hardness of the steel materials was determined on the as-received steel surface and on the top of the chromized surface with a Vickers microindenter (Buehler, Model 1600−6305) at a 200 g-f load (HV0.2) according to standardisation ASTM E384 [37]. An average value and a standard deviation of each sample were calculated from 4 measurements.
Nanoindentation
Nanoindentation measurements were performed on the polished cross-sections of the chromized steel materials to determine the hardness profiles of the chromized layer and the underlying steel substrate. The experiments were carried out with a nanoindenter (Hysitron TI900 Triboindenter, Minneapolis, MN) equipped with a Berkovich indenter. A 14 × 4 indent grit with a 5 µm spacing between each indent was created to ensure that the measurements are taken from both the steel substrate and the chromized layer. All indentation measurements were performed at a maximum load of 5 mN with a 500 µN/s loading rate. The hardness values were determined using the method developed by Oliver and Pharr [38]. The tip area function was calibrated with a quartz reference sample prior to the measurements.
Morphological and Elemental Analysis
The morphological characterization of the chromized steel materials was carried out using a scanning electron microscope (SEM, Hitachi S4800, Japan) with an accelerating voltage of 20 kV. The Distribution of constituent elements at the interface of the chromized layer and the steel substrate was determined with energy dispersive spectroscopy (EDS, AMETEK) by employing elemental line scanning analysis on the cross-section. Subsequently, the cross-sections were etched with a 2% nital solution to expose microstructural differences. The cross-sectional morphology of the etched chromized steel materials was analysed with both optical (Nikon Labophot-2, Tokyo, Japan) and SEM. The surface topography of chromized steel materials was determined with optical profilometry as described in Section 2.2.
Results
Microstructural and Compositional Analysis
Two types of surface morphologies were observed on Cr-1095, Cr-52100, Cr-A2, Cr-D2 and Cr-M2 steel materials, as shown in Figure 2: (1) densely-packed granular structures with visible grain boundaries on Cr-1095 and Cr-52100 steel surfaces, Figure 2a,b; and (2) porous, network-like structures on Cr-A2, Cr- D2 and Cr-M2 tool steel surfaces, Figure 2c,d,e. Surface topography of 65 × 65 µm2 regions measured with optical profilometry revealed that the depth of the pores (peak to valley) of Cr-A2 and Cr-D2 is on average <1 µm, Figure 3a,b, while the depth of the pores of Cr-M2 is <0.5 µm, Figure 3c. Surface topography of the larger 300 × 300 µm2 regions shows varying chromized texture of different steel substrates, Figure S2. Cr-1095 and Cr-M2 steels have relatively smooth surface while the chromized surface texture of Cr-52100, Cr-A2 and Cr-D2 steels appears to be coarser.
Figure 2 Optical images of chromized surfaces of (a) Cr-1095, (b) Cr-52100, (c) Cr-A2, (d) Cr-D2 and (e) Cr-M2 steel materials. Size of the scale bar is 2 µm.
Figure 3 Surface topographies and corresponding surface profiles of chromized (a) Cr-A2, (b) Cr-D2 and (c) Cr-M2 steel materials.
The cross-sectional SEM images of the chromized steel materials after etching along with EDS elemental line scans are shown in Figure 4. Chromizing process resulted in chromized layers with various thicknesses for different steel materials (see Supplementary Figure S3). In all chromized steel samples, the chromized layer had much higher content of Cr than Fe. In the chromized layer, the Cr content decreases gradually towards the interface with the steel substrate while the content of Fe increases proportionally in the opposite direction. The sharp change of the Cr and Fe content at the interface correlates well with the microstructural transition and was used to estimate the thickness of the chromized layer as indicated in Figure 4. On average, Cr-1095, Cr-52100 and Cr-A2 steel had a chromized layer thickness ≤24 µm. Cr-D2 tool steel had a slightly lower chromized layer thickness, ≤ 18 µm. The thickness of the chromized layer was the lowest in Cr-M2 tool steel, ≤ 12 µm. EDS mapping shows two sharp changes in the Cr and Fe content across the Cr-M2 interface, suggesting the formation of a Cr-Fe phase, Figure 4e and Figure S4b. A similar trend is also observed in Cr-1095, however, the change in the Cr and Fe content in the chromized layer is not as rapid as it is in Cr-M2.
Figure 4 Cross-sectional SEM images and EDS elemental line scans of chromized (a) Cr-1095, (b) Cr-52100, (c) Cr-A2, (d) Cr-D2 and (e) Cr-M2 steel materials. Size of the scale bar is 5 µm.
M2 and D2 tool steels contain particles which size ranges from sub-µm up to 3 µm, as observed from the cross-sectional SEM images in Figure 4d,e, Figure S3d,e and Figure S4. EDS elemental mapping revealed that these particles in D2 contain Cr, Mn and V elements, Figure S4a, while the particles in M2 contain Si, W and V elements, Figure S4b.
The cross-sectional analysis also revealed isolated pores in the chromized layers of all steel materials, Figure 4 and Figure S3. Larger pores are observed in the chromized layers of Cr-D2 and Cr-M2 closer to the interface with the steel substrate, Figure 4d,e and Figure S3d,e. The chromized layer in Cr-M2 also appears to have inner cracks and other surface flaws.
Hardness and Wear Properties
The Vickers hardness and the loop abrasion wear rate properties of the steel materials before and after chromizing are shown in Figure 5 and Table 2. The hardness of the tool steel substrates was higher than that of the 1095 and 52100 steels, Figure 5a. The hardness of D2, M2 and A2 tool steels was HV 964.7 ± 75.8, HV 904.3 ± 33.6, HV 892.6 ± 48.7, respectively while the hardness of 1095 and 52100 steels was HV 573.5 ± 20.9 and HV 750.8 ± 35.9, respectively. Chromizing increased the hardness of all steel materials. The most significant increase in the hardness was measured in Cr-1095 steel, HV 1880.8 ± 88.5, which was 3X higher than that of the substrate. Chromizing increased the hardness of 52100 steel by 2.5X to HV 1851.1 ± 131.1. The hardness of the chromized tool steels was somewhat lower than that of the chromized carbon steel: Cr-D2 steel, HV 1724.8 ± 48.5; Cr-A2 steel, HV 1670.7 ± 128.6; and Cr-M2 steel, HV 1546.7 ± 96.3.
Figure 5 Summary of mechanical testing results of the top surface of the steel materials before (untreated) and after chromizing (chromized), (a) Vickers Hardness, (b) Wear rates.
Table 2 Summary of the test samples and their properties.
Chromized steel material
Chromized layer thickness (µm)
Vickers Hardness (HV0.2)
Wear Rate (mm3/(N-m))
Hardness (GPa) (cross-section)
Untreated
Chromized
Untreated
Chromized
Chromized
1095
16–23
573.5
1880.8
5.7 × 10−3
1.4 × 10−3
17.1
52100
20–24
750.8
1851.1
5.9 × 10−3
1.0 × 10−3
17.9
A2
18–23
892
1670.7
2.1 × 10−3
1.1 × 10−3
17.1
D2
16–18
964.7
1724.8
8.6 × 10−4
6.7 × 10−3
18.8
M2
9–12
904.3
1546.7
5.3 × 10−4
7.0 × 10−3
17.5
The wear resistance of the untreated 1095 and 52100 steels was much lower than that of the untreated tool steels, however, they received more improvement by chromizing, Figure 5b and Table 2. The wear rate of Cr-52100 is 1.02 × 10−3 mm3/(N-m) which is 83% lower than the wear rate of 52100 steel before chromizing, 5.91 × 10−3 mm3/(N-m). The wear rate of the 1095 steel decreased by 76% after chromizing, from 5.70 × 10−3 to 1.35 × 10−3 mm3/(N-m). Before chromizing, the A2 steel had the highest wear rate among tool steels, 2.08 × 10−3 mm3/(N-m), and chromizing lowered the wear rate by 46% to 1.13 × 10−3 mm3/(N-m). Chromizing had less influence on the wear performance of the D2 and M2 tool steels. The wear rate of Cr-D2 was only 22% lower (8.64 × 10−4 mm3/(N-m) before chromizing vs 6.73 × 10−4 mm3/(N-m) after chromizing) while the wear rate of Cr-M2 increased 31% from 5.29 × 10−4 mm3/(N-m) before chromizing to 6.95 × 10−4 mm3/(N-m) after chromizing.
The 2D profiles of the loop abrasion wear scars on the chromized steels, Figure 6, show that the Cr layer was actually worn through on all steel samples. The thickness of the chromized layer marked on the profiles was approximated from the cross-sectional SEM images. It is clear that the worn depth is much larger than the thickness of the chromized layer on all samples. The deepest wear track was measured in Cr-1095 (∼140 µm), followed by Cr-A2 (∼125 µm), and Cr-52100 (∼115 µm). The worn depth of Cr-D2 and Cr-M2 was the lowest and very similar, ∼83 µm and ∼87 µm, respectively.
Figure 6 Average worn surface 2D profiles of chromized steels after 680 sliding cycles.
In order to probe the wear properties of the chromized layer itself, another set of loop abrasion wear experiments was performed for only 28 sliding unidirectional cycles. The average worn surface profiles of these short runs are displayed in Figure 7 and show that the wear depths of the chromized layer on all steel materials are below 20 µm. SEM images of the worn surfaces in Figure S5 showed that the chromized layer was preserved in Cr-52100 and Cr-A2, partially removed in Cr-1095 and Cr-D2, and worn through in Cr-M2. Although the surface roughness prevented precise determination of the wear rates, it can be deduced from the comparison of the average worn surface profiles that the wear performance of the chromized layer is very similar on all steel materials.
Figure 7 Average worn surface 2D profiles of chromized steels after 28 sliding cycles.
Nanoindentation hardness
Nanoindentation across the Cr-steel interface on the cross-section at a load of 5 mN confirmed that the chromized layer is harder than the substrate for all steel materials, Figure 8. Nanoindentation hardness of the steel substrates shows similar trends as the microindentation hardness in Figure 5a. The hardness of all chromized layers is in a range of ∼16 to ∼ 19 GPa which agrees with the hardness values in previous studies [17,39]. In Cr-1095, the hardness of the chromized layer increased from ∼16 to ∼18 GPa in the direction away from the interface, Figure 8a. In Cr-52100, the gradient in the chromized layer hardness appears to linearly increase towards the outer surface from ∼17 to ∼20 GPa, Figure 8b. In Cr-A2, the hardness of the chromized layer increases from ∼16 GPa at the interface to ∼18 GPa at the outer surface, Figure 8c. The hardness values of the steel substrate in Cr-D2 are more scattered due to the rougher surface caused by harder carbides, Figure 8d, similar to the substrate hardness data in Cr-M2, Figure 8e. These carbides are much harder than the steel matrix. The hardness of the chromized layer in Cr-D2 is ∼19 GPa while the hardness of the chromized layer in Cr-M2 is ∼17.5 GPa. The indentation data of the steel substrates that were <12 µm away from the interface with the chromized layer were omitted. Differences in the wear resistance between softer steel substrate and harder chromized layer resulted in a sharp topographical transition after polishing which affected the precision of the nanoindentation measurements in the close proximity to the interface.
Figure 8 Nanoindentation hardness profiles on the cross-sections of chromized (a) Cr-1095, (b) Cr-52100, (c) Cr-A2, (d) Cr-D2 and (e) Cr-M2 steels.
Discussion
Morphology of chromized steels
The differences in the surface morphology as well as in the thickness of the chromized layer are likely due to variations in the microstructures and compositions of different steel substrates. It is hypothesised that a higher content of alloying elements in M2 and D2 tool steels hindered the diffusion of Cr into the substrate which resulted in thinner chromized layers in comparison to other steel materials in this work. Based on literature, it can be assumed that M2 contains Cr, Mo, W and V carbides [40,41] while D2 contains Cr, V and Mo carbides [42,43]. Similar effect of a high content of alloying elements limiting the thickness of the chromized layer has been observed in previous studies [44–46]. Alloying elements have also been shown to hinder the diffusion of boron into the ferrous alloys [47] and steel [48] during the boronising process.
Chromizing of tool steels with high alloying content could form a multi-layered structure that consists of a Cr-Fe interlayer between the steel substrate and outermost chromized layer, as observed in Cr-M2, Figure 4e and Figure S4e. The Presence of Cr-Fe interlayer had been demonstrated in pack cementation of Cr on T9 tool steel [12] and thermal reactive diffusion of Cr on W1 tool steel [46] and Cr-V composite on D3 tool steel [45].
Unfortunately, it is not yet clear if a similar Cr-Fe layer was formed in Cr-D2 tool steel, though its chromized layer was thicker than in M2 tool steel, Figure 4d. D2 matrix already contains high concentration of chromium, Table 1, Figure S4a, which could have affected the Cr-layer thickness. The chromized layer is also composed of Fe, Mn and V elements that diffused from the matrix during the chromizing process. Although A2 tool steel and 52100 steel also contain alloying elements, they form very few carbide particles [49]. 1095 carbon steel contains no carbide particles, and the thickness of the chromized layer was comparable to A2 and 52100 steels, Figure 4a. Overall, the morphological and elemental analysis of these steel materials suggest that higher alloying content and the assumed presence of carbide particles in D2 and M2 steel matrix might have hindered the diffusion of Cr into the matrix which affected the thickness of the chromized layer.
Although the top surfaces of Cr-A2, Cr-D2 and Cr-M2 tool steels appear to be porous, Figure 2c,d,e, the depth of the porous layer is actually very small, < 1 µm, in contrast to the thickness of the chromized layers, < 24 µm. It is hypothesised that the differences in the surface structure of these chromized steel materials are due to the composition of the substrates. Higher alloying content of tool steels could have affected local diffusion rates of chromium during chromizing process. Alternatively, optimising the chromizing process parameters might allow for a smoother surface. Sub-surface pores observed in the chromized layers in all steel materials, Figure 4 and Figure S3, appear to be larger in Cr-D2 and Cr-M2 steels which suggests that the presence of these pores could be related to the steel composition. However, the sub-surface porosity could have been affected by other factors such as chromizing process parameters, chromizing mix composition or substrate surface finish. Optimizing these process parameters could mitigate pores and other flaws in the chromized layers.
In this work, the same chromizing process parameters were applied to all steel materials. Chromizing process should be optimized for different steel materials to enable thicker chromized layers [20,50,51]. This work simply presents preliminary investigation of the mechanical and tribological properties of chromized steel materials with varying composition to understand the difference in the chromizing process of these steels which could be used for further process optimization and to determine their potential in biomass pre-processing tools such as cutters or blades.
Wear and hardness of chromized steels
The results of the mechanical, tribological and morphological characterization suggest that chromizing has a more pronounced positive impact on the 1095 and 52100 carbon steels than on the A2, D2 and M2 tool steels. Chromizing only slightly improved the wear resistance of D2 tool steel and surprisingly decreased the wear resistance of the M2 tool steel even though it increased the hardness, which is counterintuitive, Figure 5. This might be explained by the morphology and composition of D2 and M2 tool steels which are notably different from 1095, 52100 and A2 steel materials. The D2 and M2 tool steels contain carbide particles which are generally harder and more wear resistant than their matrix, however, these carbides and a higher alloying content seem to limit the thickness of the chromized layer. As a result, the thinner chromized layer does not provide tribological enhancement of already wear resistant and hard D2 and M2 tool steels.
Although micro- and nanoindentation showed that the chromized layers of Cr-D2 and Cr-M2 are harder than the substrates, the differences in the wear resistance were rather small. Moreover, the results of the wear testing for a shorter period (28 sliding cycles) did not reveal any significant differences in the wear performance for the chromized layers on different steel materials. This observation suggests that the wear resistance of the chromized layers and the bulk M2 and D2 steels is very similar. It is known that an increased hardness does not necessarily enhance the wear resistance, and this is just another example. In contrast, the thicker chromized layers on Cr-A2, Cr-52100 and Cr-1095 steels notably improved both the hardness and wear resistance.
It was not expected that Cr-1095 and Cr-52100 would have higher hardness than Cr-A2 since these chromized steels have a similar thickness of the chromized layers. Untreated A2 tool steel has higher hardness than untreated 1095 and 52100 and it is counter-intuitive that Cr-A2 is softer than Cr-1095 and Cr-52100, assuming similar thickness of the chromized layers. Smaller thickness of the chromized layers in Cr-D2 and Cr-M2 as well as their lower hardness and a presence of larger sub-surface pores suggest that chromizing was less effective. More study is needed to explain this phenomenon.
Isolated pores in the chromized layers, Figure 4 and Figure S3, could have affected the hardness and tribological properties to a certain degree, especially in Cr-D2 and Cr-M2 where these pores are larger. However, the microindentation depths were relatively shallow and it is hypothesized that the apparent sub-surface porosity did not affect the hardness values significantly which can be also deduced from the smaller standard deviations in Figure 5a. On the other hand, it is quite possible that the porosity could have affected the wear properties of all chromized steel material, particularly in Cr-D2 and Cr-M2 since the pores seem to be larger.
Collectively, these findings suggest that the case-hardening chromizing of low-cost steels such as 1095 and 52100 enables them to achieve a hardness and wear resistance comparable to the more expensive tool steels such as M2 and D2. The results of this work imply that chromizing, being an affordable and high throughput process, could reduce the cost of tribological materials in applications where high hardness and wear resistance are in demand. This could be particularly impactful for biomass size reduction equipment that commonly utilise the low-cost low-alloy carbon steels [2,5,6,22,24,25]. Damage of the critical cutting tools due to excessive wear is a very costly problem and its mitigation is of a particular interest of the biofuel industry.
Conclusions
In this work, a combination of morphological characterization, multi-scale indentation hardness measurements, and wear testing was used to evaluate the tribological properties of selected carbon and tool steels treated with chromizing. The hardness and wear performance of the chromized layers were very similar for all the steel substrate materials tested here, but the thickness of the chromized layer varied depending on the steel composition. The high content of alloying elements and carbides in the D2 and M2 tool steel hindered the Cr diffusion into the steel substrate, leading to a thinner chromized layer. Chromizing demonstrated significantly improved surface hardness and abrasion resistance for the 1095 and 52100 steels, which became comparable to those of the untreated D2 and M2 tool steels. Although chromizing also increased the hardness of the D2 and M2 tool steels, the effect on the wear resistance was insignificant or even negative. This work demonstrates that low-cost carbon steels can achieve the mechanical properties of more expensive tool steels via an affordable and high throughput chromizing process, which can be beneficial in the engineering applications where both performance and cost are crucial. The results of this study could be used to optimise the chromizing process to enable greater thickness of the chromized layer with improved mechanical and tribological properties.
Acknowledgements
The authors would like to thank Jim Keiser from Oak Ridge National Laboratory (ORNL), Oyelayo O. Ajayi and George Fenske from Argonne National Laboratory for their thoughtful comments and insight, Briar Faulkner from Applied Thermal Coatings for conducting chromizing of the steel samples, and Brain Long and Caitlin Duggan from ORNL for sectioning and polishing the samples for morphological and nanoindentation studies. This research was sponsored by the Feedstock Conversion Interface Consortium (FCIC) of the Bioenergy Technologies Office, Office of Energy Efficiency and Renewable Energy, United States Department of Energy (DOE).
Author Note
This manuscript has been authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid up, irrevocable, world-wide license to publish, or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).
Disclosure statement
No potential conflict of interest was reported by the authors.
References
1. Roberts G, Krauss G, Kennedy R. Tool Steels: 5th Edition. Materials Park, Ohio: ASM International; 1998.
5. Lin L, Lanning D, Keiser JR, et al. Investigation of Cutter–Woodchip Contact Pressure in a new Biomass Comminution System. Front Energy Res. 2022;10:754811.
11. Kim HS, Yoon JH, Han JH, et al. Influence of Chromizing Treatment on the Corrosion Behavior of AISI 316 Stainless Steel in Supercritical Water Oxidation. Met Mater Int. 2004;10:83–88.
12. Liu S, Yang J, Liang X, et al. Investigation of the Preparation, Corrosion Inhibition, and Wear Resistance of the Chromized Layer on the Surfaces of T9 and SPCC Steels. Materials (Basel). 2022;15:7902.
13. Wang ZB, Lu J, Lu K. Wear and corrosion properties of a low carbon steel processed by means of SMAT followed by lower temperature chromizing treatment. Surf Coat Technol. 2006;201:2796–2801.
15. Lee JW, Wang HC, Li JL, et al. Tribological properties evaluation of AISI 1095 steel chromized at different temperatures. Surf Coat Technol. 2004;188–189:550–555.
16. Chen F-S, Lee P-Y, Yeh M-C. Thermal reactive deposition coating of chromium carbide on die steel in a fluidized bed furnace. Mater Chem Phys. 1998;53:19–27.
17. Lee JW, Duh JG. Evaluation of microstructures and mechanical properties of chromized steels with different carbon contents. Surf Coat Technol. 2004;177–178:525–531.
18. Lee J-W, Duh J-G, Tsai S-Y. Corrosion resistance and microstructural evaluation of the chromized coating process in a dual phase Fe Mn Al Cr alloy. Surf Coat Technol. 2002;153:59–66.
22. Lacey JA, Aston JE, Hernandez S, et al. Wear and why? How ash elements can help define wear profiles of biomass feedstocks. Paper No. 1901446, 2019; ASABE Annual International Meeting, Boston, MA, July 7−10, 2019 in (American Society of Agricultural and Biological Engineers, St. Joseph, MI).
24. Roy S, Lee K, Lacey JA, et al. Material Characterization-Based Wear Mechanism Investigation for Biomass Hammer Mills. ACS Sustain Chem Eng. 2020;8:3541–3546.
25. Oyedeji O, Gitman P, Qu J, et al. Understanding the Impact of Lignocellulosic Biomass Variability on the Size Reduction Process: A Review. ACS Sustain Chem Eng. 2020;8:2327–2343.
27. Dong Z, Zhou T, Liu J, et al. Effects of pack chromizing on the microstructure and anticorrosion properties of 316L stainless steel. Surf Coat Technol. 2019;366:86–96.
29. King PC, Reynoldson RW, Brownrigg A, et al. Fluidized bed CrN coating formation on prenitrocarburized plain carbon steel. J Mater Eng Perform. 2004;13:431–438.
30. Kasprzycka E, Senatorski J, Nakonieczny A, et al. Diffusion Layers Produced on Carbon Steel Surface by Means of Vacuum Chromizing Process. J Mater Eng Perform. 2003;12:693–695.
32. Kasprzycka E, Bogdański B. Properties of tool steel after hybrid treatment connecting diffusion chromizing with the PVD method. Archieves of Metallurgy and Materials. 2019;64:235–242.
33. Hu J, Zhang Y, Yang X, et al. Effect of pack-chromizing temperature on microstructure and performance of AISI 5140 steel with Cr-coatings. Surf Coat Technol. 2018;344:656–663.
34. Tan C, Zong X, Zhou W, et al. Insights into the microstructure characteriztics, mechanical properties and tribological behaviour of gas-phase chromized coating on GCr15 bearing steel. Surf Coat Technol. 2022;443:128605.
35. Standard test method for measuring abrasion resistance of materials by abrasive loop contact, ASTM G 174, ASTM International, W. Conshohocken, PA, 2003.
38. Oliver WC, Pharr GM. An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments. J Mater Res. 1992;7:1564–1583.
40. Pan FS, Wang WQ, Tang AT, et al. Phase transformation refinement of coarse primary carbides in M2 high speed steel. Prog Nat Sci: Mater Int. 2011;21:180–186.
42. Pirtovšek TV, Kugler G, Terčelj M. The behaviour of the carbides of ledeburitic AISI D2 tool steel during multiple hot deformation cycles. Mater Charact. 2013;83:97–108.
43. Hamidzadeh MA, Meratian M, Saatchi A. Effect of cerium and lanthanum on the microstructure and mechanical properties of AISI D2 tool steel. Materials Science amd Engineering: A. 2013;571:193–198.
45. Sun C, Xue Q, Zhang J, et al. Growth behavior and mechanical properties of Cr-V composite surface layer on AISI D3 steel by thermal reactive deposition. Vacuum. 2018;148:158–167.
46. Ganji O, Sajjadi SA, Yang ZG, et al. On the formation and properties of chromium carbide and vanadium carbide coatings produced on W1 tool steel through thermal reactive diffusion (TRD). Ceram Int. 2020;46:25320–25329.
Create a link to share a read only version of this article with your colleagues and friends. For more information view the Sage Journals article sharing
page.
Please read and accept the terms and conditions and check the box to generate a sharing link.
I can access personal subscriptions, purchases, paired institutional access and free tools such as favourite journals, email alerts and saved searches.
