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Cooling cracks on nitrided pins
Engineering Failure Analysis 9 (2002) 665–671 www.elsevier.com/locate/engfailanal
Cooling cracks on nitrided pins M. Godec* Institute of Metals and Technology, Lepi pot 11, PO Box 431, 1001 Ljubljana, Slovenia Received 27 April 2002; accepted 12 May 2002
Abstract A detailed examination of the surface of a pin used as a mechanical element in a ski chair-lift revealed thin cracks on the surface. The origin of these cracks was investigated and explained: we determined that the wrong nitriding parameters in combination with the cooling procedure initiated thin, thermally induced cracks. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Thermal cracks; Nitriding; White layer; Non-destructive inspection; Crack detection
1. Introduction Pins from a ski resort’s chair-lift were 70 mm in diameter, 400 mm in length and made from 42CrMo4 steel. The manufacturing process consisted of melting in an electric arc furnace, vacuum decarburisation and a conventional cast-ingot route. The slots and the holes were drilled before the heat treatment and the nitriding. The process of liquid nitriding steel has been employed in industry for many years and is used primarily to improve the wear resistance of the surface and to increase the endurance limit in fatigue [1]. The pins were nitrided using the Tenifer procedure in a salt bath at 560 C for 2 h with additional gas blowing to increase the KCN oxidation [2]. The pins were then rapidly cooled in an oil bath to a temperature of 80 C. A routine examination of the chair-lift pins showed that some of them had dangerous, thin cracks on the surface. We simulated the nitriding and thermal treatments on some identical pins and found no evidence of cracks. Additionally, the nitriding procedure was repeated on a pin with cracks and the origin of the cracks was explained on the basis of a metallographic examination.
2. Examination procedure and results Fig. 1 shows the examined pin. The pin was sectioned for the metallographic examination.
* Tel.: +386-1-4701-952; fax: +386-1-4701-939. E-mail address: [email protected] (M. Godec). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00023-7
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M. Godec / Engineering Failure Analysis 9 (2002) 665–671
Fig. 1. Pin 70 mm diameter used as a mechanical element in ski chair lift system. It is shown as cut for metallographic examination.
Fig. 2. (a) The cracks on the pin surface detected by magnetic defectoscopy using fluorescent particles. (b) Scheme showing the cracks positions on the pin surface.
M. Godec / Engineering Failure Analysis 9 (2002) 665–671
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2.1. Magnetic defectoscopy The pin’s surface was investigated using magnetic defectoscopy with black and fluorescent particles. On the pin’s surface we observed thin, parallel, longitudinal cracks up to 230 mm in length. These cracks were in groups of 3–6 at a mutual distance of 1–10 mm (Fig 2). 2.2. Metallographic analyses The cracks were also examined on small, cut specimens using an optical microscope, and no transverse cracks were observed (Fig. 3). The metallographic specimens were prepared using a classical metallographic
Fig. 3. (a) and (b) Pin surface with longitudinal cracks which have no transverse secondary branches.
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M. Godec / Engineering Failure Analysis 9 (2002) 665–671
Fig. 4. (a) Pin specimen microstructure of cross-section of surface region. Upper white layer is Fe–N–C compound with e-FeN phase and the darker area underneath is diffusion zone. Typical crack is 110 mm deep. (b) Detail of crack mouth. On the surface the crack is 1.5 mm wide. Etch: 2% Nital.
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procedure by sectioning the pin perpendicularly to the cracks [Fig. 3(a)]. The cracks started on the surface and penetrated up to 135 mm into the interior of the pin. The cracks’ mouths were 1–3 mm wide. The cracks had a tree-like appearance with the main crack sometimes moving to left and right but always proceeding in the same basic direction, which is typical for cracks propagating in a diffusion zone. Etching in nital revealed the microstructure in the surface region (Fig. 4), which consists of a thin (8–15 mm), white compound layer and underneath a thick (200–250 mm) diffusion zone. We found that all the cracks stopped in the diffusion zone. The white layer at the surface was a brittle iron nitride, which was also found at some grain boundaries near the surface. In some cases the cracks followed the brittle, white-etching nitride that formed in the prior austenite grain boundaries but it was limited in some cases and was not the reason for the cracks’ origin, as it was in the case of a chuck jaw that broke because of the presence of a brittle, whiteetching nitride surface layer [3]. In the upper white compound zone, which was very hard and brittle, we found a lot of porosity. The reason for this porosity was the differences between the flow of iron outside and the flow of nitrogen inside the metal. When the complex iron compound is formed in a salt bath [4] the conditions for a porous compound layer are fulfilled. We found the compound layer to be 80% porous, so the concentration of Na4[Fe(CN)6] was approximately 0.8% in the salt bath [4]. The cracks usually start in the pores. In Fig. 5 is a scanning electron image of the upper compound layer. The compound layer is full of thin cracks with lengths of less than 0.03 mm. The already-nitrided specimen was nitrided once again following the usual nitriding procedure. In Fig. 6 the microstructure of a twice-nitrided specimen is shown. The previous crack is now filled with a white compound layer. So it is clear that the cracks must have appeared after the nitriding. 2.3. Microhardness measurements Microhardness measurements showed that the thickness of the diffusion zone was between 200 and 250 mm. In Fig. 7 the Vickers microhardness measurements performed on the cross-section of a pin specimen are shown.
Fig. 5. Very thin cracks in the compound porous layer (scanning electron image). Etch: 2% Nital.
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Fig. 6. Microstructure of a pin specimen with a detected crack which was once again nitrided. The crack was filled with a white compound layer. Etch: 2% Nital.
Fig. 7. Vickers hardness (0.3 kg load) as a function of depth below surface for nitrided pin.
M. Godec / Engineering Failure Analysis 9 (2002) 665–671
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3. Conclusions We metallographically examined some pins that were parts from a ski resort’s chair-lift system. The surface defectoscopic analyses showed that some of them had dangerous, thin cracks on the surface. The depth of the cracks was smaller than, or similar to, the depth of the nitriding layer. The nitriding layer, with a depth between 200 and 250 mm, consisted of a diffusion layer and a compound layer. The compound layer was found to be porous due to the unoptimised parameters of the nitriding procedure. Additionally, some micro-cracks were found in the compound layer, which were the initiating points for the cracks. By an additional nitriding of an already-nitrided pin we showed that a compound layer was formed in the cracks. In the examined specimens all the cracks were free of any compound layer and so it is logical, therefore, to conclude that the cracks appeared after the nitriding, probably during the too rapid cooling of the nitrided pins. It seems that the temperature was much lower, the cooling too rapid and the temperature differences between the core and the surface were to high. The result was a stress that caused the cracks to begin at the micro-cracks in the pores.
References [1] [2] [3] [4]
US patent 3,022,204. Uhl, RC. ASM handbook, vol. 4, heat treating. USA: ASM International; 1991. p. 410–9. Stedfelf RL. ASM metals handbook, vol. 11. Failure analyses and prevention. 9th ed. Metals Rank (OH): ASM; 1986. p. 575. Finnern B, Jo¨nsson R. Waermebehandlung von Werkzeugen und Berkzeugen und Bauteilen. Muenchen: Carl Hanser Verlag; 1969. p. 91–3.
Cooling cracks on nitrided pins M. Godec* Institute of Metals and Technology, Lepi pot 11, PO Box 431, 1001 Ljubljana, Slovenia Received 27 April 2002; accepted 12 May 2002
Abstract A detailed examination of the surface of a pin used as a mechanical element in a ski chair-lift revealed thin cracks on the surface. The origin of these cracks was investigated and explained: we determined that the wrong nitriding parameters in combination with the cooling procedure initiated thin, thermally induced cracks. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Thermal cracks; Nitriding; White layer; Non-destructive inspection; Crack detection
1. Introduction Pins from a ski resort’s chair-lift were 70 mm in diameter, 400 mm in length and made from 42CrMo4 steel. The manufacturing process consisted of melting in an electric arc furnace, vacuum decarburisation and a conventional cast-ingot route. The slots and the holes were drilled before the heat treatment and the nitriding. The process of liquid nitriding steel has been employed in industry for many years and is used primarily to improve the wear resistance of the surface and to increase the endurance limit in fatigue [1]. The pins were nitrided using the Tenifer procedure in a salt bath at 560 C for 2 h with additional gas blowing to increase the KCN oxidation [2]. The pins were then rapidly cooled in an oil bath to a temperature of 80 C. A routine examination of the chair-lift pins showed that some of them had dangerous, thin cracks on the surface. We simulated the nitriding and thermal treatments on some identical pins and found no evidence of cracks. Additionally, the nitriding procedure was repeated on a pin with cracks and the origin of the cracks was explained on the basis of a metallographic examination.
2. Examination procedure and results Fig. 1 shows the examined pin. The pin was sectioned for the metallographic examination.
* Tel.: +386-1-4701-952; fax: +386-1-4701-939. E-mail address: [email protected] (M. Godec). 1350-6307/02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S1350-6307(02)00023-7
666
M. Godec / Engineering Failure Analysis 9 (2002) 665–671
Fig. 1. Pin 70 mm diameter used as a mechanical element in ski chair lift system. It is shown as cut for metallographic examination.
Fig. 2. (a) The cracks on the pin surface detected by magnetic defectoscopy using fluorescent particles. (b) Scheme showing the cracks positions on the pin surface.
M. Godec / Engineering Failure Analysis 9 (2002) 665–671
667
2.1. Magnetic defectoscopy The pin’s surface was investigated using magnetic defectoscopy with black and fluorescent particles. On the pin’s surface we observed thin, parallel, longitudinal cracks up to 230 mm in length. These cracks were in groups of 3–6 at a mutual distance of 1–10 mm (Fig 2). 2.2. Metallographic analyses The cracks were also examined on small, cut specimens using an optical microscope, and no transverse cracks were observed (Fig. 3). The metallographic specimens were prepared using a classical metallographic
Fig. 3. (a) and (b) Pin surface with longitudinal cracks which have no transverse secondary branches.
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M. Godec / Engineering Failure Analysis 9 (2002) 665–671
Fig. 4. (a) Pin specimen microstructure of cross-section of surface region. Upper white layer is Fe–N–C compound with e-FeN phase and the darker area underneath is diffusion zone. Typical crack is 110 mm deep. (b) Detail of crack mouth. On the surface the crack is 1.5 mm wide. Etch: 2% Nital.
M. Godec / Engineering Failure Analysis 9 (2002) 665–671
669
procedure by sectioning the pin perpendicularly to the cracks [Fig. 3(a)]. The cracks started on the surface and penetrated up to 135 mm into the interior of the pin. The cracks’ mouths were 1–3 mm wide. The cracks had a tree-like appearance with the main crack sometimes moving to left and right but always proceeding in the same basic direction, which is typical for cracks propagating in a diffusion zone. Etching in nital revealed the microstructure in the surface region (Fig. 4), which consists of a thin (8–15 mm), white compound layer and underneath a thick (200–250 mm) diffusion zone. We found that all the cracks stopped in the diffusion zone. The white layer at the surface was a brittle iron nitride, which was also found at some grain boundaries near the surface. In some cases the cracks followed the brittle, white-etching nitride that formed in the prior austenite grain boundaries but it was limited in some cases and was not the reason for the cracks’ origin, as it was in the case of a chuck jaw that broke because of the presence of a brittle, whiteetching nitride surface layer [3]. In the upper white compound zone, which was very hard and brittle, we found a lot of porosity. The reason for this porosity was the differences between the flow of iron outside and the flow of nitrogen inside the metal. When the complex iron compound is formed in a salt bath [4] the conditions for a porous compound layer are fulfilled. We found the compound layer to be 80% porous, so the concentration of Na4[Fe(CN)6] was approximately 0.8% in the salt bath [4]. The cracks usually start in the pores. In Fig. 5 is a scanning electron image of the upper compound layer. The compound layer is full of thin cracks with lengths of less than 0.03 mm. The already-nitrided specimen was nitrided once again following the usual nitriding procedure. In Fig. 6 the microstructure of a twice-nitrided specimen is shown. The previous crack is now filled with a white compound layer. So it is clear that the cracks must have appeared after the nitriding. 2.3. Microhardness measurements Microhardness measurements showed that the thickness of the diffusion zone was between 200 and 250 mm. In Fig. 7 the Vickers microhardness measurements performed on the cross-section of a pin specimen are shown.
Fig. 5. Very thin cracks in the compound porous layer (scanning electron image). Etch: 2% Nital.
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M. Godec / Engineering Failure Analysis 9 (2002) 665–671
Fig. 6. Microstructure of a pin specimen with a detected crack which was once again nitrided. The crack was filled with a white compound layer. Etch: 2% Nital.
Fig. 7. Vickers hardness (0.3 kg load) as a function of depth below surface for nitrided pin.
M. Godec / Engineering Failure Analysis 9 (2002) 665–671
671
3. Conclusions We metallographically examined some pins that were parts from a ski resort’s chair-lift system. The surface defectoscopic analyses showed that some of them had dangerous, thin cracks on the surface. The depth of the cracks was smaller than, or similar to, the depth of the nitriding layer. The nitriding layer, with a depth between 200 and 250 mm, consisted of a diffusion layer and a compound layer. The compound layer was found to be porous due to the unoptimised parameters of the nitriding procedure. Additionally, some micro-cracks were found in the compound layer, which were the initiating points for the cracks. By an additional nitriding of an already-nitrided pin we showed that a compound layer was formed in the cracks. In the examined specimens all the cracks were free of any compound layer and so it is logical, therefore, to conclude that the cracks appeared after the nitriding, probably during the too rapid cooling of the nitrided pins. It seems that the temperature was much lower, the cooling too rapid and the temperature differences between the core and the surface were to high. The result was a stress that caused the cracks to begin at the micro-cracks in the pores.
References [1] [2] [3] [4]
US patent 3,022,204. Uhl, RC. ASM handbook, vol. 4, heat treating. USA: ASM International; 1991. p. 410–9. Stedfelf RL. ASM metals handbook, vol. 11. Failure analyses and prevention. 9th ed. Metals Rank (OH): ASM; 1986. p. 575. Finnern B, Jo¨nsson R. Waermebehandlung von Werkzeugen und Berkzeugen und Bauteilen. Muenchen: Carl Hanser Verlag; 1969. p. 91–3.