Effect of processing conditions on the corrosion performance of lasersurfacemelted AISI 440C.docx

1、Effect of processing conditions on the corrosion performance of laser surfacemelted AISI 440CEffect of processing conditions on the corrosion performance of laser surface-melted AISI 440C martensitic stainless steelC. T. Kwoka, b, K. H. Lob, F. T. Cheng, , b and H. C. Manc a Department of Electromec

2、hanical Engineering, Faculty of Science and Technology, University of Macau, Taipa, Macau, PR Chinab Department of Applied Physics, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong, PR Chinac Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hu

3、ng Hom, Kowloon, Hong Kong, PR China Received 26 July 2002; accepted 27 October 2002.; Available online 21 December 2002. Abstract: Laser surface melting of AISI 440C martensitic stainless steel was achieved using a 2.5-kW continuous wave Nd:YAG laser. The pitting corrosion behavior of laser surface

4、-melted specimens processed under different processing conditions in 3.5% NaCl solution at 23 C was studied by potentiodynamic polarization technique. The corrosion resistance of all laser surface-melted specimens was significantly improved, as evidenced by a shift from active corrosion to passivity

5、, a wide passive range and a low passive current density. The pitting potential of the laser surface-melted specimens P08-440C-25 (laser POWER=0.8 kW, scanning SPEED=25 mm/s) and P12-440C-25 (laser POWER=1.2 kW, scanning SPEED=25 mm/s) was increased to 260 and 200 mV (SCE), respectively, and was muc

6、h higher than that of the conventionally heat-treated AISI 440C. The pitting corrosion characteristics of the laser surface-melted specimens were strongly dependent on the processing conditions which resulted in different microstructures. The enhanced corrosion resistance was attributed to the disso

7、lution or refinement of carbide particles and the presence of retained austenite. The amount of carbides in the melt layer, which indirectly determine the Cr content in solid solution and hence, the corrosion resistance, was related to the amount of C remaining in solid solution and to decarburizati

8、on. The pit morphology of the laser surface-melted specimen was also studied. Author Keywords: Laser surface melting; Pitting corrosion; Martensitic stainless steel; Nd:YAG laser 1. IntroductionIn most practical applications of engineering components, materials suffer from deterioration by mechanica

9、l and/or chemical effects present in their operating environments. Martensitic stainless steels are widely used in engineering applications such as steam and water valves, pumps, turbines, compressor components, shafting, cutlery, surgical tools, bearings and plastics moulds, etc. which demand high

10、strength and high resistance to wear and corrosion. Among the martensitic stainless steels, AISI 440C has good mechanical properties (Table 1), a high chromium content (17 wt.%) and a high carbon content (1.1 wt.%). However, the corrosion resistance of AISI 440C is the lowest among the stainless gro

11、ups because of its high carbon content, which results in the precipitation of carbide phases, although its chromium content is close to that of AISI 304 austenitic stainless steel (18 wt.% Cr). Table 1. Typical mechanical properties of selected stainless steels The application of laser surface modif

12、ication to prolong the service life of engineering components exposed to aggressive environments has gained increasing acceptance in recent years. Laser surface melting (LSM) has been proven to be a promising method for improving corrosion, wear and fatigue resistances by refining, homogenizing or t

13、ransforming the microstructure of a wide range of engineering alloys 1. The superficial layer of the components is modified while the bulk properties of the substrate are preserved. A great deal of work has been done to investigate the effect of LSM on the electrochemical corrosion properties of AIS

14、I 304 2, 3, 4 and 5, 316L 4 and 310 5 austenitic stainless steels, Zeron 100 (super duplex stainless steel, UNS S32760) 4, AISI 430 ferritic stainless steel 5 and AISI 420 martensitic stainless steel 5, 6 and 7 in sodium chloride solution. The pitting corrosion resistance of laser surface-melted AIS

15、I 304 2, 3 and 5 and 316L 4 was improved because of the removal or redistribution of manganese sulfide inclusions. For laser surface-melted AISI 430 and Zeron 100, their corrosion behavior was strongly dependent on the phases formed 4 and 5. For laser surface-melted AISI 430, a higher corrosion resi

16、stance was obtained with a microstructure containing a single ferritic or austenitic phase 5. For martensitic stainless steels, LSM causes the dissolution of large carbides, refinement of the microstructure and homogenization of chemical composition 8, resulting in improvement of hardness, toughness

17、 9, wear resistance 10 and cavitation erosion resistance 6 of these steels. The corrosion behavior of laser surface-melted AISI 420 in NaCl solution 5, 6 and 7 and in H2SO4 11 was reported by several workers. The corrosion resistance of laser surface-melted martensitic stainless steels was found to

18、be highly dependent on the microstructural change which was related to processing conditions such as power density and scanning speed of the laser beam 5, 6 and 7. Escudero and Bello 7 reported that the best corrosion behavior of laser surface-melted AISI 420 was observed in the completely melted re

19、gion. On the other hand, it was reported that LSM improved the passive performance of AISI 420 and the pitting corrosion resistance in H2SO4 approached that of AISI 304 11. Retained austenite is commonly present in laser surface-melted martensitic stainless steels 10, 12, 13 and 14. Colaco and Vilar

20、 13 reported an increase in the proportion of retained austenite with decreasing power density and increasing scanning speed in the LSM of AISI 420 martensitic stainless steel. The effect of the presence and amount of retained austenite on the corrosion behavior is still a controversial issue. No ha

21、rmful effect of retained austenite on the corrosion resistance of a 13%-Cr martensitic stainless steel was observed by Kimura et al. 15. On the other hand, Kraposhin 16 reported that there was an optimum amount of the retained austenite for best resistance to anodic dissolution. Studies related to t

22、he effect of LSM on the corrosion behavior AISI 440C martensitic stainless steel are scarce in the literature. The present work, thus, aims at improving the pitting corrosion resistance of AISI 440C in 3.5% NaCl solution by LSM, and at investigating the effect of retained austenite and undissolved c

23、arbides present. The relationship between the corrosion parameters and the laser processing conditions will also be studied, aiming at finding the optimum processing parameters. 2. Experimental details2.1. Material and specimen preparationThe as-received AISI 440C (designated as AR-440C) was in anne

24、aled condition and in the form of round bar with a hardness of 260 Hv. The nominal chemical composition in wt.% was: 17% Cr; 0.75% Mo; 1% Mn; 1.1% C; 1% Si; 0.049% P; 0.03% S; and balance Fe. Hardened specimen of AISI 440C (designated as HT-440C) was achieved by conventional heat treatment in a furn

25、ace for comparison with the laser surface-melted specimens. The specimens were preheated to, and kept at, 850 C for 45 min and then heat-treated through the austenitizing temperature (1060 C) for 30 min, followed by quenching in liquid nitrogen. Tempering was achieved by keeping the specimens at 250

26、 C for 210 min, followed by air cooling. The specimens for polarization studies were machined to discs 12.7 mm in diameter and 3.2-mm thick. The surface of the specimens for LSM was sandblasted in order to reduce the reflectivity to the laser beam. Prior to polarization tests, the surface of all las

27、er surface-melted specimens was mechanically polished with 1-m diamond paste in order to keep the surface roughness consistent. The specimens were cleaned, degreased and dried before the polarization test. 2.2. Laser surface meltingLSM was carried out using a 2.5-kW CW Nd:YAG laser. The laser beam w

28、as focused onto the specimen with a BK-7 glass lens of focal length 100 mm. Preliminary trials on the laser processing parameters for feasible treatment conditions were carried out. Laser power (P) of 1.2 kW (4.2 kW/cm2) and 0.8 kW (2.8 kW/cm2) at workpiece with a laser spot size of 6 mm in diameter

29、 and beam scanning speeds (v) of 25 to 85 mm/s were used. At scanning speeds lower than 25 mm/s, the energy input from the YAG laser was too high and thermal distortion of the specimen occurred. At scanning speed higher than 85 mm/s, cracks were present in the melt layer due to very high cooling rat

30、e. Argon flowing at 15 l/min was used as shielding gas. The designations of the samples are shown in Table 2. The melt surface was achieved by overlapping of successive melt tracks at 50% track width interval. Table 2. Laser parameters for laser surface melting of AISI 440C 2.3. Metallographic and m

31、icrostructural examinationAfter LSM, the specimens were sectioned, polished and etched in acidic ferric chloride solution (25 g FeCl3, 25 ml HCl and 100 ml H2O). The microstructure and pit morphology of the laser surface-melted layers were studied by optical microscopy (OM), scanning electron micros

32、copy (SEM) and energy-dispersive X-ray analysis (EDX). The phases present were identified by X-ray diffractometry (XRD). The radiation source used was Cu K with nickel filter and generated at 40 kV and 35 mA. 2.4. Electrochemical measurementsPolarization studies were performed in 3.5% NaCl solution to investigate the electrochemical corrosion behavior of the as-received, the conventionally heat-treated and the laser surface-melted specimens. Cyclic poten