INFLUENCE OF SURFACE HEAT TREATMENT ON THERMAL FATIGUE BEHAVIORS OF HOT WORK STEEL

Transcription

1 INFLUENCE OF SURFACE HEAT TREATMENT ON THERMAL FATIGUE BEHAVIORS OF HOT WORK STEEL Yongan Min, Luoping Xu and Xiaochun Wu School of Material Science and Engineering, Shanghai University Postal address: 149 Yanchang Road, Shanghai University, ,Shanghai, China Abstract Keywords: Self-restricted thermal fatigue tests were carried out on different surface treated specimens according to Uddeholm Method with temperature range between room temperature and 700 C, in which H13 type hot work steel was used. Developments of heat checking cracks on different specimens were detected during thermal cycles. Static residual stresses on specimens in the first hundreds cycles were also measured to evaluate the changes of cyclic dynamic stresses, and to get a better understanding of thermal fatigue mechanisms. It showed that the thermally induced cyclic stress played an important role in the generation and propagation of heat checking cracks. The residual compressive stresses on H13 specimens without high hardness layer decreased gradually, and it was apt to form evenly distributed cracks. While the higher compressive stresses on H13 specimens with high hardness layer dropped quickly. The uneven distributed stresses, usually induced by a few cracks in compound layers, were the important factor for quick developed main cracks. Heat checking was an unavoidable result of the thermally induced expansion and contraction of the hot work steel. Thermal fatigue resistance was generally decreased by worsened stress distribution due to the amplified expansion and contraction between iron matrix and diffused layer or compound layer, except when higher strength and better stress condition were acquired in surface layer. Thermal fatigue, die, hot work steel, surface treatment, stress. 63

2 64 6TH INTERNATIONAL TOOLING CONFERENCE INTRODUCTION Today thermal fatigue shown as heat checking is generally the most dominating failure in hot work dies [1]. Many kinds of surface treatment or modifications have been used on hot work dies to get longer service lives and better performance. Surface processes such as gas nitriding sometimes have a negative effect on thermal fatigue, especially those which lead to a continuous compound layer in the die surface. On the other hand, those layers are always beneficial to enhance the resistance to erosion and corrosion. Therefore, selected surface treatments have been carried out on hot work dies, especially on bigger and more complicated dies. In practice, lots of die casting dies are not surface treated, except special oxidation. A more clear understanding is expected on thermal fatigue mechanism in different surface conditions, which is complicated and multiform in certain dies due to various factors collectively acting on. Thermal fatigue specimens were respectively treated by some typical surface treating process, including plasma nitriding, plasma sulfur-carbon-nitriding, boronizing and oxidizing. The surface appearances of those specimens were observed after different cycles in order to study the nucleation and propagation behavior of heat checking under thermal fatigue test. Stress on specimens after some certain cycles were also determined by X-ray to acquire more information on the changes of heat checking. EXPERIMENTAL Cylindrical thermal fatigue specimens were made with size mm according to the test method [2] of Uddeholm Company from high quality H13 steel. After quenching and tempering, they had a hardness of 47 HRC. Observed planes were finished finely on specimens. Then these specimens were treated respectively with plasma nitriding (H13-N stand for specimens without nitride layer, H13-N+ with nitride layer), plasma sulfur-carbon-nitriding (H13-SCN), and oxidizing (H13-O). The boronizing thermal fatigue specimens (H13-B) were boronized before hardening. A group of original quenched and tempered specimens (H13) were also tested for comparison. All the thermal fatigue specimens were tempered at 200 Cbefore test. In the first heat checking experiment, different groups of specimens of H13-N,H13-SCN,H13-B and H13 were compared. Their surface features are given in Table 1.

3 Influence of Surface Heat Treatmenton Thermal Fatigue Behaviorsof Hot Work Steel65 Table 1. Features in surface layers of different group specimens Specimen Compound Diffused Surface Hardness Exp. 1 Exp. 2 layer ( µm) layer ( µm) (HV0.3) H13-N O O H-13N+ iron nitride O H13-SCN complicated nitride O H13-B iron boride O O H13-O iron oxide O H O O Thermal fatigue experiments were carried out with a self-restricted thermal fatigue equipment, in which induction heater with 200 khz frequency and 2.5 kw power was used. The total system could control the thermal fatigue procedure and record the cycle number automatically. The parameters of the test were as follows, temperatures range: room temperature 700 C; heating time: 3.6 sec; holding at high temperature:1 sec; cooling time: 8 sec; holding at room temperature:1 sec.; cooling medium: water; total cycle number: Thermal fatigue test was halted at different number of cycles, i.e. 100, 200, 400, 600, 1000, 2000, Then the specimen was immersed in dilute hydrochloric acid solution for several minutes to eliminate the oxide skin for observation. The changes on surface center areas of the observed planes of different specimens were monitored carefully after those cycles with Nikon stereomicroscope, to judge the effects of surface treatments on the thermal fatigue behavior. After 3000 cycles, the specimens were cut at center cross sections and cracks developed in depth direction were observed. The specimens used in second experiment are showed in Table 1. The test method and parameters of this experiment was just the same as the above one. In the above test dilute hydrochloric acid etchant solution was used for observation, but it influenced the surface structures and cracks without doubt. So in the second experiment it was canceled. After different cycles, residual stresses in specimens surface was directly measured, by REGAKU PSP/MSF X-ray stress instrument, in the transverse and longitudinal directions. The detection parameters were as follows, characteristic X-ray: Cr Kα; diffraction plane: αfe 211; ψ angle: 0, 15, 30, 45; stress constant,

4 66 6TH INTERNATIONAL TOOLING CONFERENCE K: -297 MPa/deg. It has been pointed out that a remarkable change of the residual stress on hot work tools happens mainly in the first hundreds shots, therefore the selected cycle numbers for stress test were as follows: 0, 50, 150, 300, 600. RESULTS DEVELOPMENT OF HEAT CHECKING CRACKS After 100 thermal cycles, very fine wrinkles appeared on H13 specimens, no change on specimens could be detected in the other groups. After 200 cycles, H13-N specimens showed fine wrinkles like the H13 specimens, but more fine and even. H13-SCN and H13-B specimens both showed a few wrinkles in some certain places. After 400 cycles, mini-cracks could be seen in every group. Uniform degree of mini-cracks distribution was in the H13-N, H13, H13-B and H13-SCN specimens orderly. After 600 cycles, H13-SCN specimens showed remarkable heat checking networks, in which thinner cracks were surrounded by main cracks. It seemed as the heat checking cracks nucleated after 200 cycles, and then developed quickly after 400 cycles. From 600 to 3000 cycles, the developing feature of heat checking was that chiefly main crack grew and thinner cracks had no obvious progress (Fig. 1a). Compared to the crack development in the H13-SCN specimens, H13-B specimens showed more main cracks and had no thinner cracks after 600 cycles. These main cracks developed evenly up to the end of the test (Fig. 1b). After 600 cycles, no main cracks could be seen on H13, and H13-N specimens, especially on H13-N specimens. All of the cracks had different extent, and some main cracks formed from 600 to 3000 cycles (Fig. 1c and Fig. 1d). After the experiment, surface hardness of the different group specimens were measured and compared to the original values as showed in Table 2. The cracks developed in depth were observed on cross-sections after 3000 cycles and shown in Fig. 2. Relative statistical diagram of the cracks on different specimens, shown in Fig. 3, were used to evaluate the destructive degree. It was regarded as a crack when deeper than mm, as observed by optical microscope with magnification 100.

5 Influence of Surface Heat Treatmenton Thermal Fatigue Behaviorsof Hot Work Steel67 Table 2. Surface hardness before and after thermal fatigue test Specimen type Original hardness (HV0.3) Final hardness (HV0.3) H13-SCN H13-B H H13-N RESIDUAL STRESSES IN THERMAL CYCLES COURSE It is well-known that stresses play an important role in the development of heat checking cracks in hot-work application. It was possible to halt the thermal fatigue test and measure the static residual stresses by X-ray diffraction, although the dynamic stresses are more important in order to understand the mechanism of thermal fatigue. The measured residual stresses on the surface of different specimens after some certain thermal cycles are showen in Fig. 4. The residual stresses in subsurface below the compound layers were measured on H13-N+, H13-B and H13-O specimens, using the αfe 211 diffraction peak. The surface compound layer disturbed the diffraction peak obviously, especially on the H13-B specimens with a thick and high hardness layer. Before testing, high compressive stresses were measured on H13-N+, H13-N specimens and lower compressive stresses on H13-O and H13 specimens. Unexpectedly low compressive stress found on H13-B specimens was likely caused by the special heat treatment processes. The change of surface stresses on H13-O specimens was most similar to that on H13 specimens. On these specimens, the residual compressive stresses decreased and waved a little with increasing cycle number. The compressive stresses on H13-N+ specimens and H13-N specimens clearly decreased with increasing cycle number. On H13-N+ specimens the compressive stresses dropped rapidly and changed to a tensile condition almost between 50 to 150 cycles, while on H13-N specimens the compressive stresses decreased gradually and kept a compressive condition till the end of 600 cycles. The compressive stresses on H13-B specimens increased to some extent with increasing cycle number.

6 68 6TH INTERNATIONAL TOOLING CONFERENCE It seemed as the surface compound layer had a remarkable influence on that change. DISCUSSIONS CYCLIC DYNAMIC STRESS DURING THERMAL CYCLES Although the dynamic stresses could not be measured during thermal cycles, the change of stress was one of the most important factors which controlled the thermal fatigue progress. The static residual stresses on specimens, measured at room temperature, may help us to evaluate the dynamic stresses and obtain a better understanding on the thermally induced stressstrain behavior of the specimens. Normally there was a residual compressive stress field distributed in the surface layer of a quenched and tempered cylindrical H13 specimen, owing to the martensite transformation and temperature change. In the thermal fatigue test, an approximate steady cyclic stress condition is reached in the specimens after a certain amount cycling [4]. The original residual stresses in the specimens had an important effect on this change. Figure 5 shows a sketch of the cyclic stress change in the section of a cylindrical H13 specimen. During the heating period the surface compressive stress increased quickly, and reached a peak at the position of the highest temperature gradient (Fig. 5a), which usually happened at the end of a heating period. During this period, the plastic strain induced by the compressive stress is regarded as the original force for heat checking [5]. During the cooling period, the surface layer contracts faster than the core and obtained a tensile state along with the change of temperature gradient (Fig. 5b). The tensile stress in this period was the direct force for nucleation and propagation of the heat checking cracks. When the surface stress was measured at room temperature, the temperature gradient vanished and the surface residual stress would keep a tensile state (Fig. 5c), or return to a compressive state (Fig. 5d). This is due to various factors including the original stress, and structural changes in the specimen and experimental conditions. The original compressive stresses on the surfaces of H13 specimens would increase plastic strains mainly caused by the temperature gradients. These contracting plastic strains then decreased the compressive stresses and even led to tensile stresses on the surfaces at room temperature. H13-O specimens had a similar change of stresses as the H13 specimens. At room temperature,

7 Influence of Surface Heat Treatmenton Thermal Fatigue Behaviorsof Hot Work Steel69 surfaces with compressive stresses indicates that a longer compressive state could be kept during the cooling period. Consequently, the development of cracks might be delayed. It seemed as if iron oxide had no remarkable influence on the original stress and cyclic stress change, while the other compound layers had a notable effect. The original H13-N+ specimens had the highest compressive stresses. During thermal cycling, the dropping stresses implied a more remarkable change of structures and strains. The nitride layer with high hardness and low thermal expansion coefficient promoted the plastic deformation as well as the temperature gradient did. Without nitride layer, the H13-N specimens showed a slow decreasing of the residual compressive stresses. In the surface diffused layer of the H13-N specimens, the increased expansion coefficient amplified contractive strains while the increased strength reduced the deformation. The crack development was restrained mainly due to the increased strength and the enhanced compressive stresses. On H13-B specimens the abnormally increased compressive residual stresses maybe meant that higher tensile plastic deformation occurred during the thermal cycles due to the effect of the borides layers. NUCLEATION AND PROPAGATION OF HEAT CHECKING CRACKS ON DIFFERENT SPECIMENS Besides external thermally induced cyclic stresses, nucleation and propagation of cracks were also controlled by internal structures and mechanical properties of the specimens. The structures and properties of the surface layers were strongly influenced by cyclic temperature and cyclic stresses. The different surface treatments led to different thermal fatigue characteristics. At the beginning of the thermal fatigue test, fine and even mini-cracks appeared on H13 and H13-N specimens because of the evenly distributed plastic deformations and stresses, while less mini-cracks appeared on H13-SCN and H13-B specimens due to the high hardness compound layers. After that, the failure of compound layers caused a concentration of strain and stress. Main cracks then propagated faster and minor cracks almost stopped, as in H13-SCN and H13-B specimens. Only main cracks particularly formed in H13-B specimens. On H13 specimens main cracks formed later, and other cracks developed with the main cracks to some extent. On H13-N specimens almost no main cracks developed with priority, but all the cracks propagated equally during the test. The width of the cracks in the cross-section of

8 70 6TH INTERNATIONAL TOOLING CONFERENCE different specimens can be seen in Fig. 2. H13 specimens showed sharp main cracks while other specimens showed wider main cracks. The width actually reflects the level of expansion and constriction during the test. Surface treatment enhanced the level and widened the surface cracks due to the difference of expansion between iron matrix and the diffused layer or compound layer. This fact implied the conclusion that the expansion difference between the surface and the core caused by surface treatment, decreases the thermal fatigue resistance except when long time compressive stresses is kept in the surface layer. A few remarkable cracks were more detrimental than numerous mini-cracks appearing in a hot work application. Therefore, optimized nitriding without compound layer is regarded as the best choice in this test. The oxidation treatment had less effect on change of stress, but a special iron oxide has showed good erosion and corrosion resistance [6]. Nitriding combined with oxidation process perhaps would be a better choice for hot-work dies. CONCLUSIONS Static residual stresses measured in thermal fatigue test reflected the change of dynamic cyclic stress. Plasma nitriding specimens without compound layer kept a residual compressive stress for longer time. A compound layer seemed to weaken the compressive stress during thermal cycling. Plasma nitriding without compound layer increased thermal fatigue resistance mainly due to the effective compressive stress on the surface of the specimen. Plasma sulfur-carbon-nitriding and boronizing decreased the resistance owing to a difference of expansion coefficients between the surface and the core and the worsening of the stress condition. Continuous compound layers were apt to form unevenly remarkable cracks. ACKNOWLEDGMENTS The authors would express their thanks to the help of Uddeholm Tooling AB, the Shanghai Fifth Steel Co, and the China Spring Co. REFERENCES [1] B. KLARENFJORD and L. A. NORSTROM, in Proceeding of the 5th Pacific Rim International Conference on Tool Steels for Dies and Molds, Shanghai, April, 1998, edited by Luoping Xu, L-A. Norstrom ( Shanghai University, Shanghai,1998) p.225.

9 Influence of Surface Heat Treatmenton Thermal Fatigue Behaviorsof Hot Work Steel71 [2] L-A. NORSTORM, Met. Technology 8 (1981) p.376. [3] M. NAGASAWA and K. KUBOTA, in Proceeding of the 5th International Conference on Tooling, Leoben, Sept., 1999, edited by F. Jeglisch (Institut fur Metallkunde und Werkstoffprufung, Leoben, 1999 ) p.225. [4] S. JEAN and B. MIQUEL, in Proceeding of the 5th International Conference on Tooling, Leoben, Sept., 1999, edited by F. Jeglisch (Institut fur Metallkunde und Werkstoffprufung, Leoben, 1999 ) p.185. [5] J. H. LIU and J. ANHUI, Inst. Techno. 7(1988) p.58. [6] A. MOLINARI and M. PELLIZZARI, Surface and Coatings technology126 (2000) p31.

10 72 6TH INTERNATIONAL TOOLING CONFERENCE Figure 1. Heat checking cracks after 600 and 3000 cycles on different surface treating specimens. (a) H13-SCN specimens. (b) H13-B specimens. (c) H13 specimens. (d) H13-N specimens.

11 Influence of Surface Heat Treatmenton Thermal Fatigue Behaviorsof Hot Work Steel73 Figure 2. Features of heat checking cracks on cross section of different specimens.

12 74 6TH INTERNATIONAL TOOLING CONFERENCE Figure 3. Statistical diagram of heat checking cracks on different specimens.

13 Influence of Surface Heat Treatmenton Thermal Fatigue Behaviorsof Hot Work Steel75 (a) Stresses in transverse direction. (b) Stresses in longitudinal direction. Figure 4. Static residual stresses of different surface treated specimens after some certain thermal cycles.

14 76 6TH INTERNATIONAL TOOLING CONFERENCE Figure 5. Sketch of cyclic stress change during the experiment. (a) Stress distribution during heating period. (b) Stress distribution during heating period. (c) One type stress distribution at room temperature. (d) Other type stress distribution at room temperature.