The deposition of high wear- and corrosion-resistant alloys through Plasma Transferred Arc (PTA) is an easily automated process combining the possibility to obtain very thick coatings with high deposition speeds, low thermal distortion of the part and negligible dilution levels, thanks to a very high energy-concentration [S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology—4th edition, Addison-Wesley Publishing, 2001, p. 789–790, Scripta Materialia 37 (1997) 721, Applied Surface Science 201 (2002) 154, Surface and Coatings Technology 71 (1995) 196, Journal of Materials Processing Technology 128 (2002) 169, M. Bonacini, Plasma ad arco trasferito per riporti saldati con superleghe in polvere, “Saldatura e taglio termico verso il 2000” IIS Meeting, October 3rd 1998, Milan-Italy, Wear 250 (2001) 611, Wear 249 (2002) 846, Composites Science and Technology 58 (1998) 299]. Literature studies regard mostly wear or high temperature behaviour of the deposited alloys and plasma hardening treatment without powder [Applied Surface Science 201 (2002) 154, Surface and Coatings Technology 71 (1995) 196, Journal of Materials Processing Technology 128 (2002) 169, M. Bonacini, Plasma ad arco trasferito per riporti saldati con superleghe in polvere, “Saldatura e taglio termico verso il 2000” IIS Meeting, October 3rd 1998, Milan-Italy, Wear 250 (2001) 611, Surface and Coatings Technology 106 (1998) 156, Wear 249 (2002) 846, Surface and Coatings Technology 92 (1997) 157, Wear 181–183 (1995) 810, Wear 225–229 (1999) 1114], rather than an optimisation of process parameters, also in critical geometric configurations. An experimental campaign has been carried out on the deposition of two nickel- and a cobalt-base superalloys: Hastelloy 276, Inconel 625 and Stellite 6. Benchmarks of C-Mn steel have been chosen to test geometrical configurations that are critical for the application (corners and grooves). Specimen characterisation through liquid penetration inspection, optical- and scanning electron microscopy and microhardness tests proved that process parameters optimisation depends only on the geometrical configuration and not on the deposited alloy. This result suggests the importance of an accurate design of the reciprocal positioning and movement between the torch and the part to be coated, as a function of the deposition geometry.

Plasma Transferred Arc deposition of powdered high performances alloys: process parameters optimisation as a function of alloy and geometrical configuration

New iron based alloys were developed for industrial hardfacing via plasma transferred arc-welding (PTAW). The alloys were designed with low critical cooling rates for metallic glass formation so that high undercooling could be obtained prior to the initiation of nucleation. The result was that conventional dendritic solidification could be avoided resulting in the refinement of metallurgical grain/phase sizes through a coupled lath eutectoid growth mode. High hardness up to R c 66 was developed from the fine structure consisting of a high volume fraction of borocarbides phases. Using a Palmqvist method, high toughness up to 74.7 MPa m 1/2 was measured which could adequately be described through a crack bridging model. The effective distribution of borocarbides phases with ductile iron in a coupled lath morphology appears to be especially potent for crack bridging and is believed enabling to achieve combinations of high toughness along with high hardness. Thus, this new class of PTAW hardfacing alloys is expected to be useful for applications requiring high abrasion resistance and high impact.

High toughness high hardness iron based PTAW weld materials

Oxide dispersion-strengthened (ODS) and molybdenum-0.5 pct titanium-0.1 pct zirconium (TZM) molybdenum have excellent creep resistance and strength at high temperatures in inert atmospheres. Fracture toughness and tensile testing was performed at temperatures between − 150 °C and 450 °C to characterize 6.35-mm-thick plate material of ODS and TZM molybdenum. A transition from low fracture-toughness values (5.8 to 29.6 MPa√m) to values >30 MPa√m is observed for TZM molybdenum in the longitudinal orientation at 100 °C and in the transverse orientation at 150 °C. These results are consistent with data reported in literature for molybdenum. A transition to low fracture-toughness values ( −150 °C, while a transition to low fracture-toughness values (12.6 to 25.4 MPa√m) was observed for the transverse orientation at room temperature. The fine spacing of La-oxide precipitates which are present in ODS molybdenum results in a transition temperature that is significantly lower than any molybdenum alloy reported to date, with upper-bound fracture-toughness values that bound the literature data. A comparison of fracture-toughness values obtained using 1T, 0.5T, and 0.25T three-point bend specimens shows that a 0.5T bend specimen could be used as a subsized geometry.

Measuring the fracture toughness of molybdenum-0.5 pct titanium-0.1 pct zirconium and oxide dispersion-strengthened molybdenum alloys using standard and subsized bend specimens

A comparative assessment of iron and cobalt-based hard-facing alloy deformation using HR-EBSD and HR-DIC

Improving cavitation erosion resistance of austenitic stainless steel in liquid sodium by hardfacing - comparison of Ni and Co based deposits

Since the mechanical properties of most wear materials are generally insufficient for structural applications, hardfacing alloys have been traditionally weld deposited to provide a wear resistance surface for a base material. An important attribute of a hardfacing alloy that is subjected to high load sliding contact is the resistance to adhesive (galling) damage. Although Co-base hardfacing alloys generally possess excellent galling wear resistance, there is interest in developing cobalt-free replacement hardfacings to reduce radiation exposure costs. A laboratory galling test has been developed for weld hardfacing deposits that is a modification of the standardized ASTM G98-91 galling test procedure. The procedure for testing a weld hardfacing deposit on a softer base metal using a button-on-block configuration is described. The contact stresses for the initiation of adhesive galling damage were measured to rank the galling resistance of several commercial Fe-base, Ni-base and Co-base hardfacing alloys. Although the galling resistance of the Fe-base alloys was generally superior to the Ni-base alloys, neither system approached the excellent galling resistance of the Co-base alloys. Microstructure examinations were used to understand the micro-mechanisms for the initiation and propagation of galling damage. A physical model for the initiation and propagation of adhesive wear is used to explain the lower galling resistance for the Ni-base hardfacings and to understand the influence of composition on the galling resistance of Ni-base alloys. The composition of some Ni base hardfacings was modified in a controlled manner to quantify the influence of specific elements on the galling resistance.

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Laboratory galling tests of several commercial cobalt-free weld hardfacing alloys

Hardfacing alloys are weld deposited to provide a wear resistant surface for structural base materials. Commercial low cobalt hardfacing alloys are being evaluated to reduce plant activation levels. Since hardfacing alloys typically must be resistant to cracking to assure adequate in service performance, fracture toughness is a critical material property. Fracture toughness (K{sub IC}) measurements of Fe base, Ni-base, and Co-base hardfacing were performed in accordance with ASTM E399-90 procedure in an effort to identify a tough cobalt-free alternative. Reduced scatter in K{sub IC} data was observed for the Fe base hardfacing, and the 95% lower bound K{sub IC} values were generally higher than the Ni-base Hardfacing alloys. Preliminary crack growth data obtained during precracking indicate that the Ni-base hardfacing possess better fatigue crack growth resistance. However, none of the Fe-base or Ni-base hardfacing have K{sub IC} values that are comparable to the reference Co-base hard facing. The test specimens were machined from thick (0.5 inches) weld deposits, and the microstructures of the test specimens are compared with the more prototypic, thinner deposits. Microstructural and fractographic examinations are used to characterize the fracture mechanisms and delineate the operative toughening mechanisms. Crack deflection and crack bridging toughening mechanisms are shown to be relevant for most of the commercial hardfacing.

/pdf/fracture-toughness-testing-and-toughening-mechanisms-of-some-371kxh44fl.pdf

B.V. Cockeram

Papers

Laboratory galling tests of several commercial cobalt-free weld hardfacing alloys

Fracture toughness testing and toughening mechanisms of some commercial cobalt-free hardfacing alloys