The Hybrid Process of Low-Pressure Carburizing and Metallization (Cr + LPC, Al + LPC) of 17Crnimo7-6 and 10Nicrmo13-5 Steels

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Article The Hybrid Process of Low-Pressure Carburizing and Metallization (Cr + LPC, Al + LPC) of 17CrNiMo7-6 and 10NiCrMo13-5 Steels

Paulina Kowalczyk *, Konrad Dybowski , Bartłomiej Januszewicz , Radomir Atraszkiewicz and Marcin Makówka

Institute of Materials Science and Engineering, Lodz University of Technology, Stefanowskiego Street 1/15, 90-924 Lodz, Poland; [email protected] (K.D.); [email protected] (B.J.); [email protected] (R.A.); [email protected] (M.M.) * Correspondence: [email protected]

Abstract: This paper presents the concept of modification of physicochemical properties of steels by simultaneous diffusion saturation with carbon and chromium or aluminum. The application of a hybrid surface treatment process consisting of a combination of aluminizing and low-pressure carburizing (Al + LPC) resulted in a reduction in the amount of retained austenite in the surface layer of the steel. While the use of chromium plating and low-pressure carburizing (Cr + LPC) induced an improvement in the corrosion resistance of the carburized steels. It is of particular importance in case of vacuum processes after the application of which the active surface corrodes easily, as well as in case of carburizing of low-alloy steel with nickel, where an increased content of retained austenite in the surface layer is found after carburizing. Citation: Kowalczyk, P.; Dybowski, K.; Januszewicz, B.; Atraszkiewicz, R.; Keywords: low pressure carburizing; hybrid heat treatment; diffusion metallization Makówka, M. The Hybrid Process of Low-Pressure Carburizing and Metallization (Cr + LPC, Al + LPC) of 17CrNiMo7-6 and 10NiCrMo13-5 1. Introduction Steels. Coatings 2021, 11, 567. https://doi.org/10.3390/ Low pressure carburizing is one of the types of gas carburizing, characterized by coatings11050567 very good efficiency and cost-effectiveness in comparison to other carburizing methods. As it is commonly known, the case-hardened layers are characterized by high strength Academic Editor: Petricaˇ Vizureanu and hardness, thanks to which they are applied widely in elements working under high stress and in point contact [1,2]. To provide additional physicochemical properties of the Received: 4 March 2021 carburized layer, it is possible to modify it by introducing another alloy element [3–7]. Accepted: 11 May 2021 The creation of layers in the process of carburizing with metallization may take place for Published: 13 May 2021 the hard carbides to form on the surface [6,8]. A similar effect can be obtained by other methods such as pack cementation process [9], TRD process [10,11], or thermal diffusion of Publisher’s Note: MDPI stays neutral chromium on grey cast iron [12]. The first example is based on the formation of a carbide with regard to jurisdictional claims in layer on the steel surface during the pack cementation process-a chromium carbide layer, published maps and institutional affil- composed of Cr23C6, is formed in the area above the chrome layer. [9]. Further examples iations. relate to the occurrence of reactive diffusion for iron-based alloys. These examples are about the formation of carbide layers by diffusion of carbon from the substrate and some elements like chromium or vanadium from metallization [10–12]. Combining metallization with carburizing process can also be used to provide e.g., Copyright: © 2021 by the authors. higher corrosion resistance by employing an additive such as nickel and/or cobalt [4]. Licensee MDPI, Basel, Switzerland. However, one of the most frequently applied elements that can ensure increase corrosion This article is an open access article resistance is chromium. The use of chromium plating to increase the corrosion resistance distributed under the terms and is a procedure that is widely applied in the case of e.g., fuel cells [13], tools [14], or other conditions of the Creative Commons elements made of different metal alloys [15–20]. Attribution (CC BY) license (https:// An important aspect regarding the steel types subjected to carburizing is the content creativecommons.org/licenses/by/ of retained austenite, which has an impact on the hardness of the layer obtained. The 4.0/).

Coatings 2021, 11, 567. https://doi.org/10.3390/coatings11050567 https://www.mdpi.com/journal/coatings Coatings 2021, 11, 567 2 of 10

high content of retained austenite results in decreased strength properties, such as wear- resistance or abrasion-resistance [19,21]. In order to enable the martensitic transformation to take place, in case of steel types with increased chromium [22] and carbon content (or the carburized layer alone) it is necessary to apply thermal processing that encompasses subzero treatment during quenching [23,24]. Introduction of aluminum as alloy element ensures an increase in the temperature of the end of martensitic transformation above 0 degrees, which impacts the reduction in the amount of retained austenite in the processed steel, for the carbon content of even up to about 0.8%. Such carbon concentrations are equal to surface carbon concentrations obtained in steel after carburizing. Thus, it provides a possibility to quench such steel in oil or in inert gases, without having to use subzero treatment. Current literature references do not indicate the use of a metallization process with a low-pressure carburizing (LPC) process to improve carburized layers by reducing the amount of residual austenite or enriching them with new physicochemical properties, e.g., increasing corrosion resistance. The surface layer alloying treatment by the metallization process proposed in this paper can signiﬁcantly enhance the previously used carburized layers and contribute to the development of a new hybrid thermochemical treatment. This paper presents the results of testing two typical carburizing steels used in the aerospace industry. These steels were subjected to aluminizing and chromium plating in combination with low-pressure carburizing. Chromium plating was used to increase the corrosion resistance of these steels, and aluminum plating was used to reduce the amount of retained austenite.

2. Materials and Methods The aim of the research was to show that there is a possibility of modifying the surface properties of the steel after carburizing to increase its corrosion resistance and to reduce the amount of retained austenite content. Increasing the corrosion resistance of steel can be achieved by increasing the content of chromium. The reduction in the amount of retained austenite is caused by the increased aluminum content. Since carburizing steels contain low content of these elements, a metallization process was applied to increase it. Therefore, diffusion metallization was combined with low-pressure carburizing in one process, and then the properties of heat-treated steels were examined.

2.1. Materials and Specimen Preparation The studies within the scope of formation of the carburized surface layer in combination with diffusion saturation with metals were carried out on two typical steel grades subjected to carburizing: 17CrNiMo7-6 and 10NiCrMo13-5, the chemical composition of which is presented in Table1. The compositions of steel were established by X-ray analysis performed using a WDXRF spectrometer (ThermoScientiﬁc company-ARLPerfom’X model, Waltham, MA, USA). The carbon content in those steel grades was determined using the infrared ration energy absorption method by means of a CS-200 analyzer (Leco company, St. Joseph, MI, USA).

Table 1. Chemical composition of 10NiCrMo13-5 and 17CrNiMo 7-6 steels.

Steel Grade C [%] Mn [%] Cr [%] Ni [%] Mo [%] Si [%] 17CrNiMo7-6 0.19 0.53 1.72 1.59 0.32 0.32 10NiCrMo13-5 0.16 0.53 1.02 3.24 0.24 0.24

The test specimens were cubes of dimensions 12 × 12 × 12 mm3. The surface of samples was ﬁrst ground on silicon carbide emery paper with grade from 180 to 1200 and then one face of each sample was polished with silicon oxide-based slurry. A chrome coating was applied to one part of the samples and a layer of aluminum to the other to obtain a diffusion metallization of the steel. The third part of the samples was intended for the reference carburizing process, without metallization. Coatings 2021, 11, 567 3 of 10

2.2. Aluminum and Chromium Coating Application Application of coatings for diffusion metallization was performed using the medium frequency power supply, 150 kHz-MFMS (Medium frequency magnetron sputtering). The samples were subjected to etching stages followed by the application of chromium or aluminum coatings, respectively. The parameters of the individual stages are presented in Table2.

Table 2. Parameters of magnetron sputtering stages for aluminum and chromium coatings.

Etching Stage Potential of Argon Pressure Argon Flow f Etching Time t 1 e - Polarization [V] pe [Pa] [sccm] [min] −800 2 25 10 - Deposition of Chromium Coating Power on the Potential on the Argon Pressure Argon Flow f Time of 2 Magnetron Cr Charge [V] pCr [Pa] [sccm] Deposition [min] PCr [kW] −50 pulsed 0.5 25 115 2 × 0.5 Deposition of Aluminium Coating Power on the Potential on the Argon Pressure Argon Flow f Time of 3 Magnetron Al Charge [V] pAl [Pa] [sccm] Deposition [min] PAl [kW] −50 pulsed 0.5 25 30 1.5

The thickness of the obtained chrome coating was gCr = 700 nm, while the thickness of the aluminum coating was gAl = 500 nm.

2.3. Heat Treatment In the ﬁrst stage of the process, the samples were subjected to diffusion metallization in accordance with the parameters given in Table3. Then, directly after metallization, low-pressure carburizing at the same temperature was applied. This process consisted of alternating steps of boost and diffusion. In the boost step, the carburizing atmosphere is dosed into the furnace chamber. Its thermal decomposition and adsorption of carbon atoms on the steel surface take place. The carburizing atmosphere was made of a mixture of gases: acetylene, ethylene, hydrogen, in the 2:2:1 volume proportions. After each boost step, a diffusion step is applied. At this stage, no process gases are dosed into the furnace chamber, only carbon is transported from the surface of the sample to its core. Detailed carburizing parameters are presented in Table3.

Table 3. Parameters of annealing and carburizing process.

Lp. Stages Temp. [◦C] Time [min] Furnace Pressure [Pa] Pumping 1 - 20 10 down 2 Heating up 1050 - 10 3 Annealing 1050 140 10 4 Boost 1050 4 300−800 5 Diffusion 1050 10 10 6 Boost 1050 2 300−800 7 Diffusion 1050 17 10 8 Boost 1050 2 300−800 9 Diffusion 1050 34 10 10 Boost 1050 2 300−800 11 Diffusion 1050 31 10 12 Cooling down 860 20 10 13 Quenching 50 - 1.2 × 106 Coatings 2021, 11, 567 4 of 10

After carburizing, initial cooling down under vacuum was applied, with an isothermal stop in that temperature with. Next, cooling in gas (nitrogen) took place down to the ambient temperature. In case of samples after the process of carburizing with chromium plating, the samples were additionally subjected to sub-zero treatment in liquid nitrogen to limit the amount of retained austenite. After carburizing and quenching, tempering was ◦ applied in the temperature of Tt = 150 C, during tt = 2 h. The applied method of obtaining hybrid layers was described in detail in the patent application [3].

2.4. Microstructure and Hardness Measurement Metallographic observations were carried out by means of scanning electron mi- croscopy with the use of a HITACHI S-3000N electron microscope (Hitachi, Tokyo, Japan). The tests were performed on cross-sections transverse to the surface of the treated steel. Metallographic specimens were made on silicon carbide sandpaper grades from 180 to 1200, and then polished and etched with the Mi1Fe reagent. After quenching and tempering, measurements of hardness distribution in the surface layer of the steel types processed were performed by Vickers’ method under a load of F = 9.81 N.

2.5. Phase Identification Identification of phases in the carburized layers of steel obtained was carried out by means of the low angle X-ray diffraction method using a PANAnalytical Empyrean X-ray diffractometer (Malvern Panalytical, Malvern, Worcestershire, UK) with a cobalt anode X-ray tube. Scanning took place in θ-θ configuration, for 2θ angles within the ◦ ◦ ◦ range of 20 –85 , with steps of 0.05 and time per step tc = 10 s. Based on the examination presented, the quantitative phase identification of retained austenite for layers carburized in combination with the metallization process (hybrid process), and without such combination, was established as well.

2.6. Examination of the Chemical Composition of the Surface Layer The chromium and aluminum concentration gradients, as well as the measurement of the depth of diffusion of these elements, were performed by means of the quantitative depth proﬁling (QDP) method, using optical emission spectrometry with glow discharge on a LECO GDS-850A device. The measurement parameters for the measured materials in the case of the above- mentioned methods were voltage Ug = 700 V,intensity Ig = 30 mA and pressure pg = 1.33 kPa. Measurements were made with a frequency of 50 Hz. The measurement accuracy of the method used is at the level of ±0.015%. The GDOES spectrometer (Leco company, St. Joseph, MI, USA) also measures the gradient of carbon concentration obtained in the steel surface layer as a result of the carburizing process. The measurement was carried out using a series of evaporations with mechanical removal of successive layers of material on AlOx sandpaper grade 180. The obtained carbon content and the amount of mechanically removed material allow us to determine the dependence of the carbon content on the depth. The concentration gradients of individual elements were determined from the surface towards the core of the tested materials. The measuring accuracy for the carbon concentration proﬁle is ±0.014%.

2.7. Corrosion Resistance Examination To evaluate corrosion resistance, the diffusion layers combining the low-pressure carburizing process with chromium plating were also subjected to a test of resistance of the impact of humidity according to the PN-EN ISO 6270-2:2006 standard [25], in the ◦ temperature of tw = 40 C and humidity of W = 98%. To determine the degree of corrosion, the weight of the corrosion products was measured using the gravimetric method for the samples after the tests of resistance to Coatings 2021, 11, x FOR PEER REVIEW 5 of 10

To evaluate corrosion resistance, the diffusion layers combining the low-pressure carburizing process with chromium plating were also subjected to a test of resistance of the impact of humidity according to the PN-EN ISO 6270-2:2006 standard [25], in the temperature of tw = 40 °C and humidity of W = 98%. Coatings 2021, 11, 567 To determine the degree of corrosion, the weight of the corrosion products5 ofwas 10 measured using the gravimetric method for the samples after the tests of resistance to moisture lasting 222 h. The samples after the corrosion tests were weighed and then moisturewashed in lasting 30 mL 222 of h.acetone The samplesusing an after ultrasonic the corrosion cleaner. testsWashing were time weighed t = 10 andmin. then This washedallowed in the 30 removal mL of acetone of loose using corrosion an ultrasonic products. cleaner. The samples Washing were time thoroughlyt = 10 min. dried This and allowedreweighed. the removalIn the next of loosestage, corrosion the rest of products. the corroded The samplesmaterial werewas thoroughlyremoved from dried the andsurface reweighed. by sandbl Inasting the next under stage, the the pressure rest of theof p corrodedp = 0.3 MPa material and with was the removed use of grain from with the a gradation of 80 µm. The samples were again washed in an ultrasonic cleaner and surface by sandblasting under the pressure of pp = 0.3 MPa and with the use of grain with a gradationweighed.of 80 µm. The samples were again washed in an ultrasonic cleaner and weighed.

3.3. Results Results 3.1.3.1. Metallographic Metallographic Observations Observations and and Identiﬁcation Identification of of Phases Phases ObservationObservation of of metallographic metallographic sections sections for for the the samples samples enabled enabled the the evaluation evaluation of of the the atmorphologyatmorphology ofof thethe layers layers created. created. TheseThese layerslayers havehave aa structurestructure typicaltypical of of steel steel after after carburizing:carburizing: martensite martensite + + retained retained austenite. austenite. No No presence presence of of carbides carbides or or other other formations formations waswas found found inin thethe surface layer structure structure of of sa samplesmples made made of of 10NiCrMo13-5 10NiCrMo13-5 steel steel after after the thehybrid hybrid process process of chromium of chromium plating plating (Figure (Figure 1a)1 anda) and aluminum aluminum coating coating (Figure (Figure 1b)1 withb) withlow lowpressure pressure carburizing carburizing (Cr (Cr + +LPC LPC and and Al Al + LPC processes).processes). Meanwhile,Meanwhile,carbide carbide formationsformations were were found found in in the the 17CrNiMo7-6 17CrNiMo7-6 steel steel after after the the hybrid hybrid process process of of carburizing carburizing withwith chromium chromium plating plating (Cr (Cr + + LPC) LPC) (Figure (Figure1 c).1c). No No compounds compounds were were found found in in the the surface surface layerlayer of of the the same same steel steel subjected subjected to a hybridto a hybrid process process of aluminum of aluminum coating andcoating low-pressure and low- carburizingpressure carburizing (Al + LPC) (Al (Figure + LPC)1d). (Figure In the 1d). steels In afterthe steels the Cr after + LPC the process,Cr + LPC it process, is difﬁcult it is todifficult reveal theto reveal structure the ofstructure the surface of the layer surf (Figureace layer1a,c), (Figure due to 1a,c), the increaseddue to the corrosion increased resistancecorrosion resultingresistance from resulting the increased from the chromium increased content.chromium A reductioncontent. A in reduction the amount in ofthe retainedamount austenite of retained in theaustenite near-surface in the zonenear-surface is evident zone inthe is evident steels after in the the steels Al + LPCafter process the Al + (FigureLPC process1b,d). (Figure 1b,d).

Figure 1. Martensitic microstructure of 10NiCrMo13-5 and 17CrNiMoCr 7-6 steels after: (a,c) low pressure carburizing process with chromium plating; (b,d) low pressure carburizing with aluminization. Etching performed with Mi1Fe, magniﬁcation ×500.

Low angle X-ray diffraction (Figure2) conﬁrmed that the compounds found were Cr7C3 chromium carbide and cementite. Coatings 2021, 11, x FOR PEER REVIEW 6 of 10 Coatings 2021, 11, x FOR PEER REVIEW 6 of 10

Figure 1. Martensitic microstructure of 10NiCrMo13-5 and 17CrNiMoCr 7-6 steels after: (a,c) low pressureFigure 1. carburizing Martensitic process microstructure with chromium of 10NiCrMo plating;13-5 (b and,d) low17CrNiMoCr pressure carburizing 7-6 steels after: with ( a,c) low aluminization.pressure carburizing Etching process performed with withchromium Mi1Fe, plating; magnification (b,d) low ×500 pressure. carburizing with aluminization. Etching performed with Mi1Fe, magnification ×500. Coatings 2021, 11, 567 Low angle X-ray diffraction (Figure 2) confirmed that the compounds found 6were of 10 Low angle X-ray diffraction (Figure 2) confirmed that the compounds found were Cr7C3 chromium carbide and cementite. Cr7C3 chromium carbide and cementite.

Figure 2. Diffractogram for 17CrNiMo 7-6 steel after low pressure carburizing process with FigurechromiumFigure 2.2. Diffractogram plating.Diffractogram for for 17CrNiMo 17CrNiMo 7-6 7-6steel steel after after low pressure low pressure carburizing carburizing process process with with chromium plating. chromium plating. 3.2. Quantitative Profile Analysis and Hardness Distribution 3.2.3.2. QuantitativeQuantitative ProﬁleProfile AnalysisAnalysis and and Hardness Hardness Distribution Distribution The results of examinations regarding quantitative profile analysis showed that for the sampleTheThe resultsresults of 10NiCrMo13-5 ofof examinationsexaminations andregarding regarding17CrNiMo7-6 quantitativequantitative steel, the profile profilechromium analysis analysis diffusion showed showed depth that that was forfor the sample of 10NiCrMo13-5 and 17CrNiMo7-6 steel, the chromium diffusion depth was thecomparable sample of and 10NiCrMo13-5 equal g1 = 28 andµm (Figure 17CrNiMo7-6 3a) and steel, g2 = 25 the µm chromium (Figure 3a), diffusion respectively. depthwas The comparablesamplecomparable shown and and also equal equal differedg g11= = 2828 µµminm the(Figure(Figure gradient3 a)3a) and and ofg g2concentration2 == 25 µµmm (Figure(Figure of 3 this3a),a), respectively. respectively.element-for The Thethe samplesample shown shown also also differed differed in the in gradient the gradient of concentration of concentration of this element-for of this theelement-for 17CrNiMo7- the 17CrNiMo7-6 steel the surface concentration was about c1 = 9%wt, while for the 617CrNiMo7-6 steel the surface steel concentration the surface was concentration about c = 9%wt, was while about for the c1 10NiCrMo13-5= 9%wt, while steel for it was the 10NiCrMo13-5 steel it was about c2 = 7%wt.1 In case of the Al + LPC process, the diffusion aboutdepth10NiCrMo13-5c for2 = both 7%wt. types steel In case ofit wassteel of the about was Al about +c2 LPC = 7%wt. 10 process, µm In (Figure case the diffusionof 3b). the The Al depth+ aluminum LPC forprocess, both content types the diffusionnear of steel the wasdepth about for 10bothµm types (Figure of 3steelb). The was aluminum about 10 µm content (Figure near 3b). the The surface aluminum was about contentc = 1.2near %wt the surface was about c3 = 1.2 %wt for 10NiCrMo13-5 steel and about c4 = 1.1%3 wt for 3 c 4 for17CrNiMo7-6surface 10NiCrMo13-5 was aboutsteel steel (Figure c and= 1.2 about3b). %wt 4for= 1.1% 10NiCrMo13-5 wt for 17CrNiMo7-6 steel and steel about (Figure c 3=b). 1.1% wt for 17CrNiMo7-6 steel (Figure 3b).

FigureFigure 3.3.Quantitative Quantitative profile profile of of chromium chromium and and aluminum aluminum concentration concentration for 10NiCrMo13-5for 10NiCrMo13-5 (a) and (a) 17CrNiMo and 17CrNiMo 7-6 (b )7-6 steels (b) steelsFigure after 3. Quantitative low-pressure profile carburizing of chromium processes and with al uminum(a) chromium concentration plating; ( bfor) aluminum 10NiCrMo13-5 coating. (a) and 17CrNiMo 7-6 (b) aftersteels low-pressure after low-pressure carburizing carburizing processes processes with (a )with chromium (a) chromium plating; plating; (b) aluminum (b) aluminum coating. coating. The comparison of results of measurements of carbon concentration and hardness profile performed for low-pressure-carburized layers with and without chromium showed that for the 10NiCrMo13-5 steel there are no significant differences in terms of the carbon concentration gradient (Figure4a). It is similar in the case of hardness; the profiles are

the same (Figure5a). For the 17CrNiMo7-6 steel, in view of the presence of carbides in its structure, the carbon content near the surface after the Cr + LPC process was higher (Figure4b) than for the process of low-pressure carburizing. It is reﬂected in the hardness distribution (Figure5b). The surface hardness after the Cr LPC process for this steel is Coatings 2021, 11, x FOR PEER REVIEW 7 of 10

Coatings 2021, 11, x FOR PEER REVIEW 7 of 10

The comparison of results of measurements of carbon concentration and hardness profileThe performed comparison for of low-pressure-carburiresults of measurementszed oflayers carbon with concentration and without and chromium hardness profileshowed performed that for the 10NiCrMo13-5for low-pressure-carburi steel there zedare nolayers significant with differencesand without in termschromium of the showedcarbon concentration that for the 10NiCrMo13-5 gradient (Figure steel 4a). there It is are similar no significant in the case differences of hardness; in terms the profiles of the Coatings 2021, 11, 567 carbonare the concentrationsame (Figure 5a). gradient For the (Figure 17CrNiMo7-6 4a). It is similarsteel, in in view the caseof the of presence hardness; of the carbides profiles7 of 10in areits structure,the same (Figurethe carbon 5a). Forcontent the 17CrNiMo7-6 near the surface steel, after in view the Cr of the+ LPC presence process of wascarbides higher in its(Figure structure, 4b) than the forcarbon the process content of near low-pressure the surface carburizing. after the Cr It +is LPCreflected process in the was hardness higher (Figuredistribution 4b) than (Figure for the 5b). process The surface of low-pressure hardness carburizing.after the Cr ItLPC is reflected process infor the this hardness steel is distributionhigher than (Figure after LPC 5b). due The toto surface thethe presencepresence hardness ofof carbides.carbides.after the Cr The LPC remainder process offor thethe this hardnesshardness steel is higherdistribution than isisafter thethe sameLPCsame due inin both both to casesthe cases presence (Figure (Figur5 eb).of 5b). carbides. The The samples samples The subject remainder subject to ato hybrid aof hybrid the process hardness process of distributioncarburizingof carburizing withis the with aluminum same aluminum in both coating cases coating did (Figur notdide show 5b).not Theshow signiﬁcant samples significant differences subject differences to regardinga hybrid regarding process carbon content in comparison to sample after the low-pressure carburizing process (Figure4). The ofcarbon carburizing content with in comparison aluminum coatingto sample did afternot showthe low-pressure significant differences carburizing regarding process situation is similar for hardness distributions. The hardness proﬁle after the LPC process is carbon(Figure content4). The situationin comparison is similar to forsample hard nessafter distributions. the low-pressure The hardness carburizing profile process after the same as after the Al + LPC process (Figure5). (Figurethe LPC 4). process The situation is the same is similar as after for the hard Al +ness LPC distributions. process (Figure The 5). hardness profile after the LPC process is the same as after the Al + LPC process (Figure 5).

Figure 4. Profile distribution of carbon for 10NiCrMo 13-5 (a) and 17CrNiMo 7-6 (b) steels after low pressure carburizing Figure 4. Proﬁle distribution of carbon for 10NiCrMo 13-5 (a) and 17CrNiMo 7-6 (b) steels after low pressure carburizing Figureonly, after 4. Profile low pressure distribution carburizing of carbon with for chromium 10NiCrMo plating 13-5 (a )and and after 17CrNiMo low pressure 7-6 (b )carburizing steels after withlow pressure aluminum carburizing coating. only, after low pressure carburizing with chromium plating and after low pressure carburizing with aluminum coating. only, after low pressure carburizing with chromium plating and after low pressure carburizing with aluminum coating.

Figure 5. Hardness distribution for 10NiCrMo13-5 (a) and 17CrNiMo 7-6 (b) steels after low pressure carburizing only, Figureafter low 5. pressureHardness carburizing distribution with for chromium10NiCrMo13-5 plating (a) anandd after17CrNiMo low pressure 7-6 (b) carburizingsteels after lowwith pressure aluminum carburizing coating. only, Figure 5. Hardness distribution for 10NiCrMo13-5 (a) and 17CrNiMo 7-6 (b) steels after low pressure carburizing only, after after low pressure carburizing with chromium plating and after low pressure carburizing with aluminum coating. low pressure carburizing with chromium plating and after low pressure carburizing with aluminum coating. 3.3. Corrosion Tests 3.3. Corrosion CorrosionFor the samplesTests Tests after the hybrid process of chromium plating and low pressure carburizingFor the the samples test of resistanceafter the hybridto humidi process ty were of performed chromium acco platingrding and to the low PN-EN pressure ISO carburizing6270-2:2006 the standard test of resistance[25], resistance in the to temperature humidi humidityty were of t performedw = 40 °C and acco according inrding humidity to the ofPN-EN W = 98%. ISO ◦ 6270-2:2006The examinations standard standard showed [25], [25], inthat the the temperature corrosion resistanceof tw == 40 40 °CforC and andthe in10NiCrMo13-5 in humidity humidity of of Wsteel = 98%. 98%.and Thethe 17CrNiMoexaminations 7-6 showed steel was that significantly the corrosion improved resistance in forcomparison the 10NiCrMo13-5 to samples steel without and thethechromium 17CrNiMo plating. 7-6 7-6 steel In the was case significantlysignificantly of these samples, improved no corrosive in comparison pitting was to samplessamples found, while withoutwithout on chromiumsamples without plating. plating. chromiumIn In the the case case ofplating, of these these samples,the samples, corro nosion no corrosive corrosiveproducts pitting pittingappear was wased found, in found, significant while while on sampleson samples without without chromium chromium plating, plating, the the corro corrosionsion products products appear appeareded in in significantsignificant amounts. The observations confirm the results of the tests carried out with the weight method. The weight loss (Figure6) confirms that the corrosion processes take place faster in the case of carburizing itself. Coatings 20212021,, 1111,, xx FORFOR PEERPEER REVIEWREVIEW 88 ofof 1010

Coatings 2021, 11, 567 amounts. The observations confirm the resultss ofof thethe teststests carriedcarried outout withwith thethe weightweight8 of 10 method. The weight loss (Figure 6) confirms that the corrosion processes take place faster inin thethe casecase ofof carburizingcarburizing itself.itself.

FigureFigure 6. 6.Weight Weight loss loss for for 10NiCrMo 10NiCrMo 13-5 13-5 and and 17CrNiMo 17CrNiMo 7-6 7-6 steels steels after after test test of resistanceof resistance to humidity- to humidity-samples after washing and sandblasting. sampleshumidity-samples after washing after and washing sandblasting. and sandblasting.

3.4.3.4. Measurement Measurement of of Retained Retained Austenite Austenite TheThe samples samples subjected subjected to to the the hybrid hybrid process process of of carburizing carburizing with with aluminum aluminum coating coating exhibitedexhibited a a signiﬁcant significant decrease decrease in in retained retained austenite austenite content content near near the the surface surface (Figure (Figure7 ).7). InInIn the thethe 17CrNiMo7-6 17CrNiMo7-617CrNiMo7-6 steel, steel,steel, the thethe amount amountamount of ofof retained retaretainedined austenite austeniteaustenite was waswas reduced reducedreduced after afterafter the thethe hybrid hybridhybrid processprocess by by 14% 14% on on average, average, while while in in 10NiCrMo13-5 10NiCrMo13-5 steel steel the the amount amount of of retained retained austenite austenite decreaseddecreased by by 28% 28% in in comparison comparison to to the the standard standard carburizing carburizing process. process.

Figure 7. Retained austenite content in 17CrNiMorNiMo 7-67-6 andand 10NiCrMo13-510NiCrMo13-5 steelssteels.. Figure 7. Retained austenite content in 17CrNiMo 7-6 and 10NiCrMo13-5 steels. 4.4. Discussion Discussion BasedBased on on the the research, research, it canit can be concludedbe concluded that that the diffusionthe diffusion of chromium of chromium into the into solid the solutionsolidsolid solutionsolution improved improvedimproved the corrosion thethe corrosioncorrosion resistance resistanresistan of thece steel of the after steel carburizing, after carburizing, without limiting without thelimitinglimiting diffusion thethe diffusion ofdiffusion carbon ofof and carboncarbon without andand changing withoutwithout changing thechanging hardness thethe ofhardnesshardness the surface ofof thethe layer surfacesurface of the layerlayer steel ofof treatedthethe steelsteel in treatedtreated this way. inin Inthisthis 17CrNiMo7-6 way.way. InIn 17CrNiMo7-617CrNiMo7-6 steel, the surfacesteel,steel, thethe hardness surfacesurface washardnesshardness high (abovewaswas highhigh 800 (above(above HV) due800 toHV) the due content to the ofcontent chromium of chromium carbides, carbides but this,, butbut can thisthis be cancan eliminated bebe eliminatedeliminated by increasing byby increasingincreasing the annealingthethe annealingannealing time during timetime duringduring the chrome thethe treatmentchromechrome treatmenttreatment of this steel ofof and thisthis thus steelsteel reduce andand the thusthus concentration reducereduce thethe ofconcentration chromium.No of chromiumchromium. carbide No chromium precipitation carbide was foundprecipitation in the 10NiCrMo13-5 was found in steel, the because this steel contains less chromium than 17CrNiMo 7-6 steel, and thus the surface concentration of chromium after the Cr + LPC process was lower. The microstructure of the carburized layer has not changed-the structure shows martensite and retained austenite. The mentioned phases were only saturated with chromium. Coatings 2021, 11, 567 9 of 10

In the case of the Al + LPC process, the addition of aluminum to the steel reduced the amount of retained austenite after carburizing and quenching, and thus increased the surface hardness. The microstructure of the surface layer for both steels showed a clear decrease in the amount of retained austenite, which is desirable for carburized layers. This is most likely due to the increase in the temperature of the ﬁnal stage of the martensitic transformation, which is inﬂuenced by the aluminum content. In this case, the carbon concentration gradient and hardness distribution were also the same as after the LPC process.

5. Conclusions The conducted research shows that it is possible to increase the physic-chemical properties of low-pressure carburized layers as a result of additional saturating the surface layer of steel with a suitable type of element. In both steel grades, the saturation with aluminum and chromium did not limit carbon diffusion, nor did it change the hardness proﬁle, but gave these steels additional properties. In the case of a high surface concentration of carbon, as is the case after carburizing steel, Al + LPC process may replace the necessity subzero treatment after quenching. This is important for low-alloy steels with nickel, where an increased content of retained austenite in the surface layer is found after carburizing. While the application of the process of Cr + LPC makes it possible to increase the corrosion resistance of the steel after carburizing. This, in turn, is particularly important importance in the case of vacuum processes, after which the active surface corrodes easily.

Author Contributions: Conceptualization, P.K. and K.D.; methodology, K.D., P.K., and B.J.; investigation, P.K., B.J., R.A., and M.M.; writing—original draft preparation, P.K.; writing—review and editing, K.D. and B.J.; supervision, K.D. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the statutory activities of Institute of Materials Science and Engineering of the Lodz University of Technology. Data Availability Statement: The data presented in this study are available in this article. Acknowledgments: The authors thank Adam Rzepkowski for the hardness examinations. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Preisser, F.; Seemann, R.; Zenker, W.R. Vacuum carburizing with high pressure gas quenching—The application. In Proceedings of the International Automotive Heat Treating Conference, Puerto Vallarta, Mexico, 13–15 July 1998; pp. 135–141. 2. Kula, P.; Dybowski, K.; Wolowiec, E.; Pietrasik, R. Boost-diffusion vacuum carburizing—Process optimization. Vacuum 2014, 99, 175–179. [CrossRef] 3. Dybowski, K.; Kowalczyk, P.; Atraszkiewicz, R.; Januszewicz, B. Method for Producing Hybrid Layer on A Base From Iron or Its Alloys. Patent Application No. P. 416,797, 11 April 2016. 4. Ramqvist, L.H.; Grinder, N.O.; Sporrong, M. Enghang, Nickel and/or Cobalt-Coated Steel with Carburized Interface. U.S. Patent No. 4,013,487, 22 March 1977. 5. Kula, P.; Kaczmarek, Ł.; Dybowski, K.; Pietrasik, R.; Krasowski, M. Activation of carbon deposit in the process of vacuum carburizing with preliminary nitriding. Vacuum 2013, 87, 26–29. [CrossRef] 6. Dybowski, K.; Kowalczyk, P.; Januszewicz, B.; Batory, D. Hybrid layer type Cr/LPC. Adv. Sci. Technol. Res. J. 2017, 11, 23–27. [CrossRef] 7. Kula, P.; Pietrasik, R.; Dybowski, K.; Paweta, S.; Wolowiec, E. Properties of surface layers processed by a new, high-temperature vacuum carburizing technology with prenitriding—PreNitLPC®. Adv. Mater. Res. 2013, 452–453, 401–406. 8. Latifa, S.; Mehmooda, M.; Ahmada, J.; Aslama, M.; Ahmedb, M.; Zhangc, Z. Ni–WC composite coatings by carburizing electrodeposited amorphous and nanocrystalline Ni–W alloys. Appl. Surf. Sci. 2010, 256, 3098–3106. [CrossRef] 9. Meißner, T.M.; Montero, X.; Fähsing, D.; Galetz, M.C. Cr diffusion coatings on a ferritic-martensitic steel for corrosion protection in KCl-rich biomass co-ﬁring environments. Corros. Sci. 2020, 164, 108343. [CrossRef] 10. Biesuz, M.; Sglavo, V.M. Chromium and vanadium carbide and nitride coatings obtained by TRD techniques on UNI 42CrMoS4 (AISI 4140) steel. Surf. Coat. Technol. 2016, 286, 319–326. [CrossRef] 11. Ghadi, A.; Soltanieh, M.; Saghaﬁan, H.; Yang, Z.-G. Investigation of chromium and vanadium carbide composite coatings on CK45 steel by Thermal Reactive Diffusion. Surf. Coat. Technol. 2016, 289, 1–10. [CrossRef] Coatings 2021, 11, 567 10 of 10

12. Su, X.; Zhao, S.; Hou, J.; Yu, G.; Chen, Y.; Sun, H.; Zhang, P.; Xie, L. Formation of chromium carbide coatings on HT250 steel by thermal diffusion processes in ﬂuoride molten salt bath. Vacuum 2018, 155, 219–223. [CrossRef] 13. Lee, S.B.; Cho, K.H.; Lee, W.G.; Jang, H. Improved corrosion resistance and interfacial contact resistance of 316L stainless-steel for proton exchange membrane fuel cell bipolar plates by chromizing surface treatment. J. Power Sour. 2009, 187, 318–323. [CrossRef] 14. Jiinjen Albert Sue Unnikrishnan, C.; Vasudevan Rajagopala, N. Pillai: Multilayered Coating for Downhole Tools with Enhanced Wear Resistance and Acidic Corrosion Resistance. U.S. Patent Application No. US20150132604A1, 14 May 2015. 15. Pryor, M.J. Process for Improving Corrosion Resistant Charakteristics of Chrome Plated Aluminium and Aluminium Alloys. U.S. Patent US4111763A, 5 September 1978. 16. Feistenauer, S.; Exner Weiß, T.; Alvarez, D.; Rost, M. Functional Chromium Layer with Improved Corrosion Resistance. U.S. Patent US20160024674A1, 28 January 2016. 17. Mazzochi, J.L. Chrome Plating Process. U.S. Patent US5401379A, 28 March 1995. 18. Kim, H.J.; Kweon, Y.G. The effects of retained austenite on dry sliding wear behavior of carburized steels. Wear 1996, 193, 8–15. [CrossRef] 19. Kasprzycka, E.; Bogdanski, B. Anti-wear properties of medium carbon steel after carburizing with subsequent diffusion chromizing and PVD treatment. Tribology 2019, 6, 25–30. [CrossRef] 20. Nikolov, K.; Kaestner, P.; Klages, C.-P.; Schuhmacher, B. Low-pressure diffusion chromising of thin low-carbon steel sheet for improved surface and bulk properties. J. Alloys Compd. 2017, 692, 101–107. [CrossRef] 21. Da Silva, V.F.; Canale, L.F.; Spinelli, D.; Bose-Filho, W.W.; Crnkovic, O.R. Inﬂuence of retained austenite on short fatigue crack growth and wear resistance of case carburized steel. J. Mater. Eng. Perform. 1999, 8, 543–548. [CrossRef] 22. Hossain, R.; Pahlevani, F.; Sahajwalla, V. Effect of small addition of Cr on stability of retained austenite in high carbon steel. Mater. Charact. 2017, 125, 114–122. [CrossRef] 23. Preciado, M.; Bravo, P.M.; Alegre, J.M. Effect of low temperature tempering prior cryogenic treatment on carburized steels. J. Mater. Process. Technol. 2016, 176, 41–44. [CrossRef] 24. Baldissera, P.; Delprete, C. Effects of deep cryogenic treatment on static mechanical properties of 18NiCrMo5 carburized steel. Mater. Des. 2009, 30, 1435–1440. [CrossRef] 25. PN-EN ISO 6270-2:2006 Standard: Paints and Varnishes-Determination of Resistance to Humidity-Part 2: Procedure for Exposing Test Specimens in Condensation-Water Atmospheres. 15 December 2006. Available online: https://www.doerken-mks.de/en/ glossary-old/condensed-water-test-according-to-din-en-iso-6270-2/ (accessed on 13 May 2021).