Low-Cycle Fatigue Strength of Borocarburized 15Nicr13 Steel Piotr Dziarski*, Natalia Makuch, Michał Kulka, Daria Mikołajczak

Home , Boriding, Low hydrogen annealing

WWW.SIGMA-NOT.PL Inżynieria Materiałowa 2 (204) (2015) 69÷73 DOI 10.15199/28.2015.2.4 © Copyright SIGMA-NOT MATERIALS ENGINEERING

Low-cycle fatigue strength of borocarburized 15NiCr13 steel Piotr Dziarski*, Natalia Makuch, Michał Kulka, Daria Mikołajczak

Instytut Inżynierii Materiałowej, Politechnika Poznańska, *[email protected]

The high fatigue resistance of carburized layers is well known. Simultaneously, there is not much data referring to the fatigue strength of borided layers. Some papers showed the advantageous influence of borocarburizing process on fatigue performance. The resistance of borocarburized layers to the low- cycle fatigue was higher than the one characteristic of typical borided layer formed on medium-carbon steel. In this study, the two-step process: carburizing followed by boriding was used in order to form the borocarburized layer. The investigated material as well as the boriding parameters were adequately selected in order to improve the low-cycle fatigue strength. The borocarburized 15NiCr13 steel was examined. This material was selected because of its

advantageous carbon concentration-depth profile beneath iron borides obtained after boriding. The gas boriding in N2–H2–BCl3 atmosphere consisted of two stages: saturation with boron and diffusion annealing, alternately repeated. This treatment was carried out in order to obtain a limited amount of the brittle FeB phase in the boride zone. The low-cycle fatigue strength of through-hardened borocarburized steel was comparable to that obtained in case of through- hardened carburized specimen, which was previously investigated under the same conditions. The advantageous carbon concentration-depth profile as well as limited amount of FeB phase had a positive influence on the low-cycle fatigue strength. Therefore, the fatigue performance of borocarburized layer could approach a limit obtained for carburized layer. Key words: gas boriding, borocarburizing, microstructure, hardness, low-cycle fatigue.

Niskocyklowa wytrzymałość zmęczeniowa boronawęglanej stali 15NiCr13 Duża odporność zmęczeniowa warstw nawęglanych jest powszechnie znana. Jednocześnie nie ma zbyt wielu danych dotyczących wytrzymałości zmęcze- niowej warstw borowanych. Niektóre prace wskazywały na korzystny wpływ boronawęglania na odporność zmęczeniową. Dla warstw boronawęglanych uzyskiwano większą odporność niż dla typowych warstw borowanych otrzymywanych na stali średniowęglowej. W pracy zastosowano do wytworzenia warstwy boronawęglanej dwustopniowy proces nawęglania i borowania. Badany materiał i parametry procesu borowania zostały odpowiednio dobrane w celu polepszenia niskocyklowej wytrzymałości zmęczeniowej. Do badań użyto boronawęglaną stal 15NiCr13, na której można było otrzymać korzystny

profil stężenia węgla pod borkami żelaza po borowaniu. Borowanie gazowe w atmosferze N2–H2–BCl3 składało się z dwóch etapów: nasycania borem i wyżarzania dyfuzyjnego. Celem takiej obróbki było otrzymanie warstwy borków o ograniczonym udziale kruchej fazy FeB. Niskocyklowa wytrzymałość zmęczeniowa boronawęglanej i utwardzonej cieplnie stali 15NiCr13 była porównywalna do osiągniętej dla nawęglanej i utwardzonej cieplnie próbki, którą badano w tych samych warunkach. Korzystny profil stężenia węgla oraz ograniczony udział fazy FeB w strefie borków miały pozytywny wpływ na niskocyklową wytrzymałość zmęczeniową. Właściwości zmęczeniowe boronawęglanej stali mogły się w ten sposób zbliżyć do wartości otrzymywanych dla stali nawęglanych. Słowa kluczowe: borowanie gazowe, boronawęglanie, mikrostruktura, twardość, niskocyklowe zmęczenie.

1. INTRODUCTION layers were usually formed on previously carburized substrate. It caused the significantly lower low-cycle fatigue strength than that- Diffusion boriding being a thermochemical process is widely used obtained in case of carburized layer [15]. for production of boride-type layer. This process results in the for- The single-phase microstructure might provide the layer with mation of FeB and Fe2B needle-like microstructure on the steel’s still better properties. The production of single-phase boride layer surface. The occurrence of iron borides increases to a high degree: was possible with different methods. In case of pack boronizing hardness (up to 2000 HV), wear resistance and corrosion resistance or paste boronizing, the use of powder or paste of proper compo- [1÷7]. sition was necessary [23, 24]. Boronizing under a glow-discharge As for the main disadvantage of boriding, the brittleness of conditions also provided the layer with single-phase microstructure. borided layers needs to be mentioned. This brittleness is caused by This process was performed at reduced pressure in an atmosphere several factors. First, the iron borides (especially, FeB) have a high with an ionized gas carrier [2]. As a boron source, BCl3 was usu- hardness. Besides, a large hardness gradient exists between the ally used. The main advantage of this process was a faster produc- borided layer and the substrate. There are many methods, which can tion of the boride layer. Reduction of BCl3 content (to 4 vol. %) decrease the brittleness of the boride layers. The top three methods caused, at first, Fe2B boride formation. In these conditions, after are: obtaining a single-phase Fe2B layer [2, 6, 7], the production of boronizing at 800°C (1073 K) for 120 s, on the whole surface of multicomponent and complex borided layers [8÷16] and laser-heat the sample, a continuous layer consisting of Fe2B was observed [2]. treatment (LHT) after boriding [17÷22]. In order to form single-phase borided layer (Fe2B) during typical The borocarburizing process [12÷16] led to the formation of gas boriding, previously applied, the proper conditions of this pro- multicomponent layers (B-C) by tandem diffusion processes: pre- cess were required [25]. The single-phase structure was obtainable carburizing and boriding. These layers were characterized by im- as a consequence of two independent processes of long duration. proved properties, especially by increased abrasive wear resistance In the first step, the boriding was carried out in H2–BCl3 atmos- [12÷15] and increased low-cycle fatigue strength [14, 15] in com- phere at 900÷950°C (1173÷1223 K) for 4 h, and the two-phase parison with typical borided layers. However, the two-phase boride boride layer was produced (FeB + Fe2B). The second step consisted

NR 2/2015 INŻYNIERIA MATERIAŁOWA 69 in the diffusion annealing of produced two-phase layer in H2 atmosphere at 900°C (1173 K) for 2 h. During this annealing the boron diffuses from the FeB boride along the grain boundaries. This diffusion caused a gradual reduction of FeB, and an increase of Fe2B amount in the boride layer. In the paper [16], an alternative method of gas boronizing was used for the formation of a borocarburized layer. The two-stage boriding process was carried out in N2–H2–BCl3 atmosphere. The obtaining the boride layers of limited percentage of the brittle FeB phase was the main motivation of the proposed method. In this study, the same two-stage process was applied in order to improve the low-cycle fatigue strength of borocarburized layer. The importance of the carbon concentration-depth profile beneath iron borides for low-cycle fatigue was previously reported [15]. There- fore, 15NiCr13 steel was investigated. In case of this material, an advantageous carbon profile after boriding could be obtained (relatively low carbon content beneath iron borides and adequate thickness of the layer). Fig. 1. The devices used for gas boriding Rys. 1. Urządzenia stosowane do borowania gazowego 2. EXPERIMENTAL PROCEDURE

15NiCr13 steel was investigated. Its chemical composition was pre- by boron and its diffusion inside the carburized substrate. Boron sented in Table 1. The ring-shaped specimens (external diameter ca. trichloride was added to N2–H2 atmosphere for 15 minutes. In case 20 mm, internal diameter 12 mm and height 12 mm) were used for of boride layers, formed on steels during gas boriding in H2–BCl3 the study. atmosphere [6, 12÷15, 20÷22], the content of BCl3 below 5 vol. % The parameters of the diffusion processes were shown in Ta- was the most advisable. The content higher than 5 vol. % provided ble 2. The borocarburizing consisted of two tandem diffusion pro- the iron borides with a larger porosity. The considerable quantity of cesses: precarburizing and boriding. The borocarburized layer was ferrous and ferric chlorides in atmosphere was the reason for deteri- formed on 15NiCr13 steel. First, the gas carburizing was carried out oration in the quality of the layer. However, the higher BCl3 content in controlled carburizing atmosphere at 930°C (1203 K). Cracked in relation to H2 accelerated the saturation by boron and its diffu- methanol with propane–butane gas was used in order to produce sion. As a consequence, the thicker iron borides layers were obtained the carburizing atmosphere. Carbon potential CFeC was controlled [25]. Therefore, in this study, the relatively high BCl3 addition (in by means of dew-point measuring system and by pure Fe–C foils relation to the hydrogen) was used (about 8.6 vol. %). Simultane- carburized until equilibrium with atmosphere was obtained. Carbon ously, the addition of BCl3, in relation to the entire atmosphere used content in Armco iron corresponded to carbon potential of atmos- (N2–H2–BCl3), was relatively low (2.3 vol. %). Therefore, the pro- phere. The specimens were carburized at carbon potential of 1.2% C cess proved to be more economical in BCl3 consumption. during 3 hours. After carburizing the specimens were slowly cooled The second stage (diffusion annealing) had to reduce, or elimi- in used atmosphere. nate FeB phase. The addition of BCl3 was switched off for next 15 After carburizing, gas boriding was carried out with the usage minutes. In this stage, there was only diffusion of boron towards of devices presented in Figure 1. The specimens were put into the the carburized substrate. Reducing the supply of boron from the quartz tube. Prior to heating, the system was checked by vacuum atmosphere resulted in a reduction of boron concentration in the meter, in order to ensure that the air had been removed by the vacu- material and affected the phase composition of boride layer. This um pump. Next, the flow of nitrogen was activated and the heating cycle was repeated four times for two hours. The amount of BCl3 process was started. After the furnace had reached a temperature resulted from its temperature, which was measured by thermometer of 910°C (1183 K), a gas mixture of N2–H2 was fed through the resistor PT100 located on the gas cylinder. The scheme of BCl3 ad- quartz tube at a flow rate of 100 l/h. This atmosphere consisted of dition during the boronizing process was presented in Figure 2. The

75 vol. % N2 and 25 vol. % H2. The gases of high purity were ap- diffusion process continued for 2 hours, then the boriding finished plied (nitrogen 6.0 and hydrogen 6.0). and the specimens were cooled in a nitrogen atmosphere.

Then, the addition of BCl3 was realized during the two-stage pro- Through-hardening was carried out after diffusion borocarbur- cess of boronizing. The first step consisted in diffusion saturation izing. The specimens were quenched in oil from 850°C (1123 K) and tempered at 150°C (423 K). The microstructure of polished and etched cross-sections of the borocarburized layer was observed by an light microscope (LM). Table 1. Chemical composition of material used, wt % The hardness profiles through formed layers were determined in Tabela 1. Skład chemiczny stosowanego materiału, % mas. the polished cross-sections of specimens. For microhardness meas- Material C Cr Ni Mn Si urements (Vickers method) the apparatus ZWICK 3212 B was applied. The tests were made under the loading P = 0.1 KG (about 15NiCr13 0.14 0.73 2.87 0.41 0.30 0.981 N). A hydraulic pulsator MTS 810 of maximal load 100 kN was used during the low-cycle fatigue tests. The borocarburized and through- hardened specimen was investigated. The main elements of the Table 2. The parameters of diffusion processes testing equipment were as follows: the working elements (beam Tabela 2. Parametry procesów dyfuzyjnych with force gauge, upper head with clamp, supporting pillars, frame, Diffusion Diffusion carburizing boriding piston with working head and clamp); electronic control; control Material Type of process panel; master switch; personal computer with software (Station CFeC Temp. Time Temp. Time wt % °C h °C h Manager, Basic Test Ware); hydraulic system with pump. The tests were carried out in order to obtain a complete fracture of the speci- 15NiCr13 Borocarburizing 1.2 930 3 910 2 men or to determine the fatigue life. The specimens were put under

70 INŻYNIERIA MATERIAŁOWA ROK XXXVI

Fig. 2. The scheme of BCl addition during two-stage gas boronizing 3 Fig. 3. The scheme of specimen position during the low-cycle fatigue process in N –H –BCl atmosphere 2 2 3 test; 1 – thrust washers (41Cr4 steel), 2 – specimen Rys. 2. Schemat dodawania BCl podczas dwuetapowego procesu borow- 3 Rys. 3. Schemat mocowania próbki podczas próby niskocyklowego ania gazowego w atmosferze N –H –BCl 2 2 3 zmęczenia; 1 – podkładki (stal 41Cr4), 2 – próbka

radial compression. The compressive and bending stresses were generated. The scheme of the specimen position was presented in Figure 3. The relatively high load F of 1.6÷4.4 kN had been used, hence the fatigue cracks occurred after a low number of cycles. The compressive force F was modulated with a sinusoidal applied force of frequency 10 Hz and an amplitude A of 0.8÷2.2 kN. The load and the amplitude were generated with the help of working head move- ment. During the test, the deflection v was registered continuously. The contacting ekstensometer MTS 634.31F-24 (axial – multiple gage length) was used for the measurements of deflection. 3. RESULTS AND DISCUSSION

The microstructure of diffusion borocarburized layer formed on 17CrNi6-6 steel with the proposed method of gas boriding, presented in this study, was shown in Figure 4. The two-stage process in N2–H2–BCl3 atmosphere was applied. The high ratio of BCl3 to the hydrogen (about 1:10.6) was used during the saturation by boron. It seemed that the microstructure at the surface consisted of Fig. 4. Microstructure of borocarburized layer formed on 15NiCr13 steel; 1 – FeB iron borides, 2 – Fe B iron borides, 3 – carburized sub- FeB borides (1) and Fe B phase (2). A darker FeB zone was visible. 2 2 strate; two-step gas boriding in N –H –BCl atmosphere; BCl addi- The thickness of this phase was relatively small comparing to the 2 2 3 3 tion: 8.6 vol. % in H –BCl mixture results of the typical continuous boriding. Beneath the iron borides, 2 3 the carburized substrate (3) consisting of pearlite was observed. In Rys. 4. Mikrostruktura warstwy boronawęglanej wytworzonej na stali 15NiCr13; 1 – borki żelaza FeB, 2 – borki żelaza Fe B, 3 – nawęglone case of 17CrNi6-6 steel [15], the alloyed cementite occurred be- 2 neath iron borides. Although the high carbon potential was used podłoże; dwustopniowy proces borowania gazowego w atmosferze N –H –BCl ; dodatek BCl : 8,6% obj. w mieszaninie H –BCl during carburizing, the alloyed cementite presence was not detected 2 2 3 3 2 3 beneath the boride zone formed on 15NiCr13 steel. The hardness profiles, obtained after two-stage boriding of carburized steel, were presented in Figure 5. The profiles with and in the core of steel. The through-hardened borocarburized speci- without through-hardening were compared. The highest values of men was characterized by the higher hardness of the core (about hardness were accompanied by the presence of an iron boride zone 500 HV). The use of two-step diffusion process (borocarburizing)

(FeB + Fe2B). The hardness at the surface was shown in Figure 5a. caused the decrease in the hardness gradient between the surface In the boride zone of borocarburized 15NiCr13 steel, the micro- and the substrate in comparison with typical borided layers formed hardness of about 1300÷1650 HV was obtained. These values were on medium-carbon steels [12÷16]. characteristic of Fe2B phase. The depth of FeB zone was too small It is well known that carburized layers are characterized by high in order to measure the hardness of this phase. Beneath iron borides, fatigue resistance. There is not much information referring to the the hardness decreased to values typical of carburized layer with or fatigue strength of borided layers. The influence of boronizing on without heat treatment. In case of through-hardened borocarburized the fatigue strength is ambiguous [26], because it depends on many layer a higher hardness (about 850 HV) was observed in compari- factors: boriding method, boriding parameters, chemical composi- son with the layer directly after borocarburizing (about 400 HV). tion of borided steel, heat treatment after boriding, and the defects The hardness profiles through the entire diffusion layers were of the layers. presented in Figure 5b. The gradual decrease in the hardness of Some results showed the advantageous influence of borocarbur- carburized zone was observed. In case of borocarburized specimen izing process on fatigue performance. The resistance of borocarbur- without heat treatment, the hardness decreased to about 250 HV ized layer to the low-cycle fatigue was higher than the one char-

NR 2/2015 INŻYNIERIA MATERIAŁOWA 71 a)

b) Fig. 6. Results of low-cycle fatigue during radial compression of borocarburized and through-hardened 15NiCr13 steel Rys. 6. Wyniki niskocyklowego zmęczenia przy promieniowym ściskaniu boronawęglanej i utwardzonej cieplnie stali 15NiCr13

Fig. 5. Microhardness profiles of borocarburized layers formed on 15NiCr13 steel: a) at the surface, b) through all diffusion layers; two-step gas boriding in N2–H2–BCl3 atmosphere; BCl3 addition: 8.6 vol. % in H2–BCl3 mixture Rys. 5. Profile mikrotwardości warstw boronawęglanych na stali 15NiCr13: a) przy powierzchni, b) w całym przekroju warstw dyfuzyjnych; dwustopniowy proces borowania gazowego w atmosferze

N2–H2–BCl3; dodatek BCl3 8,6% obj. w mieszaninie H2–BCl3

Fig. 7. First crack of borocarburized and through-hardened sample acteristic of typical borided layer formed on medium-carbon steel during low-cycle fatigue test [14]. However, the fatigue performance of carburized layer was bet- Rys. 7. Pierwsze pęknięcie boronawęglanej i utwardzonej cieplnie próbki ter. Research data reported the importance of carbon concentration- podczas próby niskocyklowego zmęczenia depth profile beneath iron borides for low-cycle fatigue strength [15]. The carbon concentration-depth profile beneath iron borides influenced the crack propagation in borocarburized layer formed on 17CrNi6-6 and 15NiCr13 steels. More advantageous profile was Table 3. The results of low-cycle fatigue tests obtained in case of borocarburized layer formed on 15NiCr13 steel. Tabela 3. Wyniki prób niskocyklowego zmęczenia Lower carbon content beneath iron borides decreased the amount Complete fracture First crack of alloyed cementite and retained austenite in comparison with the Material and type of treatment of specimen number of cycles same layer formed on 17CrNi6-6 steel. It reduced crack growth, number of cycles caused by shear fracture, and resulted in better fatigue resistance. Simultaneously, the increased amount of cementite beneath iron Borocarburized and through- borides caused the worse residual stress distribution [25]. There- hardened 15NiCr13 steel 36,241 36,270 (this study) fore, the lower carbon content beneath iron borides advantageously Carburized and through-hardened influenced low-cycle fatigue strength. This was the reason for using 37,706 37,710 15NiCr13 steel in this study. 17CrNi6-6 steel [22] The results of low-cycle fatigue, during the radial compression of through-hardened borocarburized specimen, were presented in Figure 6. The complete fracture of the investigated specimen was observed after 36,270 cycles. The detailed analysis of obtained pro- 4. CONCLUSIONS files (Fig. 7) showed that the first crack occurred at the end of test, before the complete fracture of the specimen. The results of the pre- Two-step process: carburizing followed by boriding was applied to sented study were comparable to those-obtained in case of through- the formation of borocarburized layer on 15NiCr13 steel. The mate- hardened carburized specimen, which was investigated under the rial, as well as the parameters of boriding, were adequately selected. same conditions (Tab. 3). Probably, the limited amount of the brittle The carburized layer formed on 15NiCr13 steel was characterized FeB phase was the reason for the relatively high low-cycle fatigue by advantageous carbon concentration-depth profile, in respect strength of borocarburized layer. of resistance to fatigue. Additionally, the two-stage gas boriding

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