ICMCTF2003 Session A1-1: Coatings to Resist High Temperature Corrosion and Wear
Monday, April 28, 2003 10:30 AM in Room Sunset
A1-1-1 New Coatings for Continuous Casting Rolls
A. Sanz (Danieli & C, Italy)
Rolls in steel industry withstand very high loads, thermal cycling (and thermal fatigue) and severe environmental aggression. In the case of coated rolls, it has been observed that under high load the coating might fail by rolling fatigue process instead of wear. The fatigue behavior of coatings is not only depending on the Hertzian contact pressure alone but it is highly influenced by the machine characteristics. Rolls are mainly exposed to: a) thermal fatigue due to high temperature thermal cycling (this is the most critical parameter), b) mechanical stresses due to the high bending stresses associated with the slab weight, c) corrosion-oxidation, d) Wear due to abrasive solid mold fluxes and oxide scale on the slab surface. The reciprocating tribological test allows characterizing failures due to the thermal and mechanical stresses. The test is divided in two main phases that are repeated until maximum test time is reached. Phase 1 has a duration of 60 minutes at 400°C, 10Hz, 30N (Hertzian pressure 1300 MPa) and phase 2 lasts 15 minutes at 900 °C, 1Hz, 30 N (Hertzian pressure 1300 MPa). The cycle was repeated until the end of the coating lifetime or interrupted after 8 hours. A second test for a shorter period of time (50% of the total test duration) was carried out. All tests are in ambient air without any lubricant. The coatings were also characterized by abrasive wear tests. This measurement is performed with an abrasive solution (SiC particles in water). The position of the sphere relative to the sample and the contact load must be constant. The sphere made in 100 Cr6 steel, the test load and speed are 0,4 N and 10 cm/sec respectively Abrasive tests last 1 minute. Several coating obtained with different coating techniques (laser cladding, thermal spray + chemical slurry and welding) were characterized to validate The objective of this work is design coatings able to cast at least 2,000,000 tons with a coated roll.
A1-1-2 Thermal Fatigue Cracking of Surface Engineered Hot Work Tool Steels
A. Persson (Karlstad University, Sweden); S. Hogmark (Uppsala University, Sweden); J. Bergström (Karlstad University, Sweden)
Thermal fatigue cracking is an important life-limiting failure mechanism in die casting tools. It is observed as a network of fine cracks on the surface exposed to thermal cycling. The crack network degrades the surface quality of the tool and, consequently, the surface of the casting. Surface engineered materials are today successfully applied to improve the erosion and corrosion resistance. However, their resistance against thermal fatigue is not fully explored. In this work, a selection of hot work tool steel grades was surface modified and experimentally evaluated in a dedicated thermal fatigue simulation test. The surface modifications included boriding, nitriding and Toyota diffusion (CrC) and physically vapour deposited coatings (CrC, CrN and TiAlN), both as single-layers and deposited after nitriding (duplex-treatment). Untreated specimens of each tool steel grade were used as references. The test is based on cyclic induction heating and internal cooling of hollow cylindrical test rods. The surface strain is continuously recorded through a non-contact laser speckle technique. Generally, all surface treatments decrease the resistance against surface cracking of surface engineered tool steels, as compared to the reference materials. The reason is that the fatigue cracking is negatively influenced by the modification of the mechanical properties of the tool material that occurs during the engineering process. However, in a relatively early stage, PVD coating may retard the fatigue cracking whereas, later on, it shows a tendency to facilitate the cracking.
A1-1-4 Application of A High Temperature Self-lubricating Composite Coating on Steam Turbine Components
W. Wang (Elliott Turbomachinery Company)
A high temperature self-lubricating composite coating has been successfully applied on steam turbine governor valve lift rods that work at 540°C and are subject to metal-to-metal frictional contact. The coating, NASA designation PS304, is a chrome-oxide based plasma sprayed coating with silver and BaF2/CaF2 eutectic compound embedded to function as solid lubricants. An inspection of the coated lift rods, after 8500 hours operation, showed that the surfaces were well protected. Further examinations indicated that a lubricous glaze, containing chrome-oxide, silver, & barium/calcium fluorides, had formed on the rod surfaces and effectively performed protection there. Lab evaluation of the coating, which had been exposed at 540°C for 150 hours, demonstrated that the coating was thermally stable. The mechanisms of the solid lubricants transferring to the counterface under different conditions have been discussed.
A1-1-5 Residual Stress in HVOF Thermally Sprayed Thick Deposits
J. Stokes, L. Looney (Materials Processing Research Centre (MPRC), Ireland)
Due to the nature of the HVOF thermal spray process, residual stresses in thick deposits is a significant problem. During deposition by thermal spraying, the deposit undergoes extreme and rapid changes of temperature, resulting in residual stress generation. Since the residual stress-state that evolves during deposition is largely dependent on the thermal conditions to which the system has been subjected, namely quenching stresses during deposition and cooling stresses post-deposition, precise predictions of these phenomena are essential, if a thick deposit is to be thermally sprayed. The paper investigates an analytical technique, used to predict residual stress generation in thermally sprayed deposits based on geometric and material properties. Residual stress results are compared to experimental results (XRD and Hole Drilling Method). A noticeable change in stress-state from tensile to compressive stress as the deposit thickness increased during HVOF spraying is analysed in terms of quenching and cooling stresses.