1、c Department of Industrial and Systems Engineering, The Hong Kong Polytechnic University, Hung 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
2、using a 2.5-kW continuous wave Nd:YAG laser. The pitting corrosion behavior of laser surface-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 surface-melted specimens
3、was significantly improved, as evidenced by a shift from active corrosion to passivity, a wide passive range and a low passive current density. The pitting potential of the surface-melted specimens P08-440C-25 （ POWER=0.8 kW, scanning SPEED=25 mm/s） and P12-440C-25 （ POWER=1.2 kW, scanning SPEED=25
4、mm/s） was increased to 260 and 200 mV （SCE）, respectively, and was much higher than that of the conventionally heat-treated AISI 440C. The pitting corrosion characteristics of the surface-melted specimens were strongly dependent on the processing conditions which resulted in different microstructure
5、s. The enhanced corrosion resistance was attributed to the dissolution 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
6、the amount of C remaining in solid solution and to decarburization. The pit morphology of the 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 co
7、mponents, materials suffer from deterioration by mechanical 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 to
8、ols, bearings and plastics moulds, etc. which demand high 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 res
9、istance of AISI 440C is the lowest among the stainless groups 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 select
10、ed stainless steels The application of surface modification 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 an
11、d fatigue resistances by refining, homogenizing or transforming 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 LS
12、M on the electrochemical corrosion properties of AISI 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 pitt
13、ing corrosion resistance of surface-melted AISI 304 2, 3 and 5 and 316L 4 was improved because of the removal or redistribution of manganese sulfide inclusions. For surface-melted AISI 430 and Zeron 100, their corrosion behavior was strongly dependent on the phases formed 4 and 5. For surface-melted
14、 AISI 430, a higher corrosion resistance 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
15、improvement of hardness, toughness 9, wear resistance 10 and cavitation erosion resistance 6 of these steels. The corrosion behavior of surface-melted AISI 420 in NaCl solution 5, 6 and 7 and in H2SO4 11 was reported by several workers. The corrosion resistance of surface-melted martensitic stainles
16、s steels was found to be highly dependent on the microstructural change which was related to processing conditions such as power density and scanning speed of the beam 5, 6 and 7. Escudero and Bello 7 reported that the best corrosion behavior of surface-melted AISI 420 was observed in the completely
17、 melted region. 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 surface-melted martensitic stainless steels 10, 12, 13 and 14. Colaco and V
18、ilar 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. N
19、o harmful 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
20、to the 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 undissolv
21、ed carbides present. The relationship between the corrosion parameters and the 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 anneal
22、ed 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 furnac
23、e for comparison with the 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 C for 2
24、10 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 beam. Prior to polarization tests, the surface of all surface-melted sp
25、ecimens 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. surface meltingLSM was carried out using a 2.5-kW CW Nd:YAG laser. The laser beam was focused onto the specim
26、en with a BK-7 glass lens of focal length 100 mm. Preliminary trials on the processing parameters for feasible treatment conditions were carried out. power （P） of 1.2 kW （4.2 kW/cm2） and 0.8 kW （2.8 kW/cm2） at workpiece with a spot size of 6 mm in diameter and beam scanning speeds （v） of 25 to 85 mm
27、/s were used. At scanning speeds lower than 25 mm/s, the energy input from the YAG 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 rate. Argon flowing at 15 l/min was used as shielding
28、 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. parameters for surface melting of AISI 440C 2.3. Metallographic and microstructural examinationAfter LSM, the specimens were sectio
29、ned, 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 surface-melted layers were studied by optical microscopy （OM）, scanning electron microscopy （SEM） and energy-dispersive X-ray analysis （EDX）. The phases pr
30、esent 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 surface-melted specimens. Cyclic poten
