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Low-cycle fatigue strength of borocarburized 15NiCr13 steel

Piotr Dziarski*, Natalia Makuch, Michał Kulka, Daria MikołajczakInstytut Inżynierii Materiałowej, Politechnika Poznańska, *piotr.dziarski@put.poznan.pl

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.

Inżynieria Materiałowa 2 (204) (2015) 69÷73DOI 10.15199/28.2015.2.4© Copyright SIGMA-NOT

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MATERIALS ENGINEERING

1. INTRODUCTION

Diffusion boriding being a thermochemical process is widely used for production of boride-type layer. This process results in the for-mation of FeB and Fe2B needle-like microstructure on the steel’s surface. The occurrence of iron borides increases to a high degree: hardness (up to 2000 HV), wear resistance and corrosion resistance [1÷7].

As for the main disadvantage of boriding, the brittleness of borided layers needs to be mentioned. This brittleness is caused by several factors. First, the iron borides (especially, FeB) have a high hardness. Besides, a large hardness gradient exists between the borided layer and the substrate. There are many methods, which can decrease the brittleness of the boride layers. The top three methods are: obtaining a single-phase Fe2B layer [2, 6, 7], the production of multicomponent and complex borided layers [8÷16] and laser-heat treatment (LHT) after boriding [17÷22].

The borocarburizing process [12÷16] led to the formation of multicomponent layers (B-C) by tandem diffusion processes: pre-carburizing and boriding. These layers were characterized by im-proved properties, especially by increased abrasive wear resistance [12÷15] and increased low-cycle fatigue strength [14, 15] in com-parison with typical borided layers. However, the two-phase boride

layers were usually formed on previously carburized substrate. It caused the significantly lower low-cycle fatigue strength than that-obtained in case of carburized layer [15].

The single-phase microstructure might provide the layer with still better properties. The production of single-phase boride layer was possible with different methods. In case of pack boronizing or paste boronizing, the use of powder or paste of proper compo-sition was necessary [23, 24]. Boronizing under a glow-discharge conditions also provided the layer with single-phase microstructure. This process was performed at reduced pressure in an atmosphere with an ionized gas carrier [2]. As a boron source, BCl3 was usu-ally used. The main advantage of this process was a faster produc-tion of the boride layer. Reduction of BCl3 content (to 4 vol. %) caused, at first, Fe2B boride formation. In these conditions, after boronizing at 800°C (1073 K) for 120 s, on the whole surface of the sample, a continuous layer consisting of Fe2B was observed [2]. In order to form single-phase borided layer (Fe2B) during typical gas boriding, previously applied, the proper conditions of this pro-cess were required [25]. The single-phase structure was obtainable as a consequence of two independent processes of long duration. In the first step, the boriding was carried out in H2–BCl3 atmos-phere at 900÷950°C (1173÷1223 K) for 4 h, and the two-phase boride layer was produced (FeB + Fe2B). The second step consisted

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in the diffusion annealing of produced two-phase layer in H2 atmos-phere at 900°C (1173 K) for 2 h. During this annealing the boron diffuses from the FeB boride along the grain boundaries. This dif-fusion 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 (rela-tively low carbon content beneath iron borides and adequate thick-ness of the layer).

2. EXPERIMENTAL PROCEDURE

15NiCr13 steel was investigated. Its chemical composition was pre-sented in Table 1. The ring-shaped specimens (external diameter ca. 20 mm, internal diameter 12 mm and height 12 mm) were used for the study.

The parameters of the diffusion processes were shown in Ta-ble 2. The borocarburizing consisted of two tandem diffusion pro-cesses: precarburizing and boriding. The borocarburized layer was formed on 15NiCr13 steel. First, the gas carburizing was carried out in controlled carburizing atmosphere at 930°C (1203 K). Cracked methanol with propane–butane gas was used in order to produce the carburizing atmosphere. Carbon potential CFeC was controlled by means of dew-point measuring system and by pure Fe–C foils carburized until equilibrium with atmosphere was obtained. Carbon content in Armco iron corresponded to carbon potential of atmos-phere. The specimens were carburized at carbon potential of 1.2% C during 3 hours. After carburizing the specimens were slowly cooled in used atmosphere.

After carburizing, gas boriding was carried out with the usage of devices presented in Figure 1. The specimens were put into the quartz tube. Prior to heating, the system was checked by vacuum meter, in order to ensure that the air had been removed by the vacu-um pump. Next, the flow of nitrogen was activated and the heating process was started. After the furnace had reached a temperature of 910°C (1183 K), a gas mixture of N2–H2 was fed through the quartz tube at a flow rate of 100 l/h. This atmosphere consisted of 75 vol. % N2 and 25 vol. % H2. The gases of high purity were ap-plied (nitrogen 6.0 and hydrogen 6.0).

Then, the addition of BCl3 was realized during the two-stage pro-cess of boronizing. The first step consisted in diffusion saturation

by boron and its diffusion inside the carburized substrate. Boron trichloride was added to N2–H2 atmosphere for 15 minutes. In case of boride layers, formed on steels during gas boriding in H2–BCl3 atmosphere [6, 12÷15, 20÷22], the content of BCl3 below 5 vol. % was the most advisable. The content higher than 5 vol. % provided the iron borides with a larger porosity. The considerable quantity of ferrous and ferric chlorides in atmosphere was the reason for deteri-oration in the quality of the layer. However, the higher BCl3 content in relation to H2 accelerated the saturation by boron and its diffu-sion. As a consequence, the thicker iron borides layers were obtained [25]. Therefore, in this study, the relatively high BCl3 addition (in relation to the hydrogen) was used (about 8.6 vol. %). Simultane-ously, the addition of BCl3, in relation to the entire atmosphere used (N2–H2–BCl3), was relatively low (2.3 vol. %). Therefore, the pro-cess proved to be more economical in BCl3 consumption.

The second stage (diffusion annealing) had to reduce, or elimi-nate FeB phase. The addition of BCl3 was switched off for next 15 minutes. In this stage, there was only diffusion of boron towards the carburized substrate. Reducing the supply of boron from the atmosphere resulted in a reduction of boron concentration in the material and affected the phase composition of boride layer. This cycle was repeated four times for two hours. The amount of BCl3 resulted from its temperature, which was measured by thermometer resistor PT100 located on the gas cylinder. The scheme of BCl3 ad-dition during the boronizing process was presented in Figure 2. The diffusion process continued for 2 hours, then the boriding finished and the specimens were cooled in a nitrogen atmosphere.

Through-hardening was carried out after diffusion borocarbur-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).

The hardness profiles through formed layers were determined in the polished cross-sections of specimens. For microhardness meas-urements (Vickers method) the apparatus ZWICK 3212 B was ap-plied. The tests were made under the loading P = 0.1 KG (about 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 testing equipment were as follows: the working elements (beam with force gauge, upper head with clamp, supporting pillars, frame, piston with working head and clamp); electronic control; control panel; master switch; personal computer with software (Station Manager, Basic Test Ware); hydraulic system with pump. The tests were carried out in order to obtain a complete fracture of the speci-men or to determine the fatigue life. The specimens were put under

Table 1. Chemical composition of material used, wt %Tabela 1. Skład chemiczny stosowanego materiału, % mas.

Material C Cr Ni Mn Si

15NiCr13 0.14 0.73 2.87 0.41 0.30

Table 2. The parameters of diffusion processes Tabela 2. Parametry procesów dyfuzyjnych

Material Type of processDiffusion carburizing Diffusion

boridingCFeC wt %

Temp. °C

Time h

Temp. °C

Time h

15NiCr13 Borocarburizing 1.2 930 3 910 2

Fig. 1. The devices used for gas boridingRys. 1. Urządzenia stosowane do borowania gazowego

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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, pre-sented 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 bo-ron. It seemed that the microstructure at the surface consisted of FeB borides (1) and Fe2B phase (2). A darker FeB zone was visible. The thickness of this phase was relatively small comparing to the results of the typical continuous boriding. Beneath the iron borides, the carburized substrate (3) consisting of pearlite was observed. In case of 17CrNi6-6 steel [15], the alloyed cementite occurred be-neath iron borides. Although the high carbon potential was used during carburizing, the alloyed cementite presence was not detected beneath the boride zone formed on 15NiCr13 steel.

The hardness profiles, obtained after two-stage boriding of car-burized steel, were presented in Figure 5. The profiles with and without through-hardening were compared. The highest values of hardness were accompanied by the presence of an iron boride zone (FeB + Fe2B). The hardness at the surface was shown in Figure 5a. In the boride zone of borocarburized 15NiCr13 steel, the micro-hardness of about 1300÷1650 HV was obtained. These values were characteristic of Fe2B phase. The depth of FeB zone was too small in order to measure the hardness of this phase. Beneath iron borides, the hardness decreased to values typical of carburized layer with or without heat treatment. In case of through-hardened borocarburized layer a higher hardness (about 850 HV) was observed in compari-son with the layer directly after borocarburizing (about 400 HV).

The hardness profiles through the entire diffusion layers were presented in Figure 5b. The gradual decrease in the hardness of carburized zone was observed. In case of borocarburized specimen without heat treatment, the hardness decreased to about 250 HV

Fig. 2. The scheme of BCl3 addition during two-stage gas boronizing process in N2–H2–BCl3 atmosphereRys. 2. Schemat dodawania BCl3 podczas dwuetapowego procesu borow-ania gazowego w atmosferze N2–H2–BCl3

Fig. 3. The scheme of specimen position during the low-cycle fatigue test; 1 – thrust washers (41Cr4 steel), 2 – specimenRys. 3. Schemat mocowania próbki podczas próby niskocyklowego zmęczenia; 1 – podkładki (stal 41Cr4), 2 – próbka

Fig. 4. Microstructure of borocarburized layer formed on 15NiCr13 steel; 1 – FeB iron borides, 2 – Fe2B iron borides, 3 – carburized sub-strate; two-step gas boriding in N2–H2–BCl3 atmosphere; BCl3 addi-tion: 8.6 vol. % in H2–BCl3 mixtureRys. 4. Mikrostruktura warstwy boronawęglanej wytworzonej na stali 15NiCr13; 1 – borki żelaza FeB, 2 – borki żelaza Fe2B, 3 – nawęglone podłoże; dwustopniowy proces borowania gazowego w atmosferze N2–H2–BCl3; dodatek BCl3: 8,6% obj. w mieszaninie H2–BCl3

in the core of steel. The through-hardened borocarburized speci-men was characterized by the higher hardness of the core (about 500 HV). The use of two-step diffusion process (borocarburizing) caused the decrease in the hardness gradient between the surface and the substrate in comparison with typical borided layers formed on medium-carbon steels [12÷16].

It is well known that carburized layers are characterized by high fatigue resistance. There is not much information referring to the fatigue strength of borided layers. The influence of boronizing on the fatigue strength is ambiguous [26], because it depends on many factors: boriding method, boriding parameters, chemical composi-tion of borided steel, heat treatment after boriding, and the defects of the layers.

Some results showed the advantageous influence of borocarbur-izing process on fatigue performance. The resistance of borocarbur-ized layer to the low-cycle fatigue was higher than the one char-

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acteristic of typical borided layer formed on medium-carbon steel [14]. However, the fatigue performance of carburized layer was bet-ter. Research data reported the importance of carbon concentration-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 obtained in case of borocarburized layer formed on 15NiCr13 steel. Lower carbon content beneath iron borides decreased the amount of alloyed cementite and retained austenite in comparison with the same layer formed on 17CrNi6-6 steel. It reduced crack growth, caused by shear fracture, and resulted in better fatigue resistance. Simultaneously, the increased amount of cementite beneath iron borides caused the worse residual stress distribution [25]. There-fore, the lower carbon content beneath iron borides advantageously influenced low-cycle fatigue strength. This was the reason for using 15NiCr13 steel in this study.

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-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-sented study were comparable to those-obtained in case of through-hardened carburized specimen, which was investigated under the same conditions (Tab. 3). Probably, the limited amount of the brittle FeB phase was the reason for the relatively high low-cycle fatigue strength of borocarburized layer.

4. CONCLUSIONS

Two-step process: carburizing followed by boriding was applied to the formation of borocarburized layer on 15NiCr13 steel. The mate-rial, as well as the parameters of boriding, were adequately selected. The carburized layer formed on 15NiCr13 steel was characterized by advantageous carbon concentration-depth profile, in respect of resistance to fatigue. Additionally, the two-stage gas boriding

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 mixtureRys. 5. Profile mikrotwardości warstw boronawęglanych na stali 15NiCr13: a) przy powierzchni, b) w całym przekroju warstw dy-fuzyjnych; dwustopniowy proces borowania gazowego w atmosferze N2–H2–BCl3; dodatek BCl3 8,6% obj. w mieszaninie H2–BCl3

Fig. 6. Results of low-cycle fatigue during radial compression of boro-carburized and through-hardened 15NiCr13 steelRys. 6. Wyniki niskocyklowego zmęczenia przy promieniowym ściskaniu boronawęglanej i utwardzonej cieplnie stali 15NiCr13

Fig. 7. First crack of borocarburized and through-hardened sample during low-cycle fatigue testRys. 7. Pierwsze pęknięcie boronawęglanej i utwardzonej cieplnie próbki podczas próby niskocyklowego zmęczenia

Table 3. The results of low-cycle fatigue testsTabela 3. Wyniki prób niskocyklowego zmęczenia

Material and type of treatment First crack number of cycles

Complete fracture of specimen

number of cycles

Borocarburized and through-hardened 15NiCr13 steel

(this study)36,241 36,270

Carburized and through-hardened 17CrNi6-6 steel [22] 37,706 37,710

a)

b)

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in N2–H2–BCl3 atmosphere was used in order to obtain a limited amount of the brittle FeB phase at the surface.

The microstructure consisted of a thin zone of FeB borides, Fe2B phase, and carburized layer beneath iron borides. Although the high carbon potential was used during carburizing, the alloyed cementite was not detected beneath the boride zone formed on 15NiCr13 steel. The borocarburized and through-hardened layer was characterized by a gradual decrease in hardness towards the core of steel. At the surface, the hardness was equal to about 1300÷1650 HV. These val-ues corresponded to Fe2B borides. The depth of FeB zone was too small to measure the hardness in this phase by the method used.

The literature data reported that carburized zone beneath iron borides has to meet the same requirements, which are characteristic of carburized layers of high fatigue resistance. In case of presented study, 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.

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