Kenneth G. Budinski

Tribological properties of titanium alloys

Abstract Titanium is often the most cost-effective corrosion-resistant material for applications in the chemical process industry that involve resistance to halides. Unfortunately, there are usually some components in these systems that involve relative sliding-titanium tribosystems — e.g. bolts and nuts, valves, piping connections, etc. Titanium and its alloys have had a reputation for poor tribocharacteristics, but detailed information on suitable counterfaces and wear specifics is scarce. This paper summarizes a study conducted on the two most widely used titanium alloys, Grade 2 commercially pure titanium and the age-hardenable Ti6Al4V. Dry sand-rubber wheel tests were conducted to assess abrasion resistance; fretting, galling and reciprocating pin-on-plane tests were conducted to determine if there is a “best” counterface for these two titanium alloys. The test results are distilled into recommendations for use of titanium alloys in tribosystems in the chemical process industry. Both alloys have poor abrasion resistance. Grade 2 pure titanium should be avoided in all titanium tribosystems and there are preferred counterfaces for the Ti6Al4V alloy, but the best metal-to-metal wear resistance is obtained when the alloy is anodized and coated with a dry film lubricant.

Resistance to particle abrasion of selected plastics

Abstract Accelerated abrasive wear of plastic parts in a piece of production machinery prompted a laboratory study to find a material with better abrasion resistance. The abrasion occurred in a machine that compacted ‘sand-like’ particles of an inorganic compound. The abrasion resistance of a wide variety of plastics and different durometer polyurethanes (21 materials) was tested with a modification of the ASTM dry-sand rubber wheel three-body abrasion test. Only one material, a polyurethane, had better abrasion resistance than the material that was currently in use. Hardness, friction and scratch tests were conducted on the test materials to try to understand the role of material properties in this type of abrasion. None of these correlated with the wear data. Previous investigators of plastic abrasion related abrasion resistance to the fracture energy and friction. The wear data developed in this study did not correlate with the specific model proposed by Ratner. However, it was possible to obtain a reasonable correlation with a deformation factor that included the friction of the abrasive on the plastic and a term that related to the energy required to deform the material plastically. A test similar to a Brinell hardness test was used to arrive at the deformation energy of the 21 test materials. The more easily the material deforms in contact with a particular abrasive, the better the abrasion resistance.

Incipient galling of metals

Abstract Incipient galling defined as “the early stages of surface deformation and adhesive interaction in metal-to-metal wear” was studied to solve seizure problems in sanitary valves. Preliminary studies aimed at evaluating the relative galling tendencies of various metal couples indicated that incipient galling can occur with most metal couples. In-depth studies on a special galling rig showed that friction is not a predictor of galling tendencies, that abrasive blasted surfaces significantly reduced galling tendencies over polished or ground surfaces and that the most galling-resistant materials have a limited plasticity (the ratio of tensile strength to yield strength is near unity). Typical materials that fulfill this criterion are hardened tool steels, hard diffusion coatings, ceramics and many cermets.

The wear resistance of diffusion treated surfaces

Abstract For many years there has been a controversy over the relative abrasion and metal-to-metal wear resistance of wear resistant platings and diffusion coatings. Does chromium plating wear better than nitrided or carburized steel? Because these surface treatments can be as thin as 3 μm and as thick as 500 μm it is difficult to assess their relative wear resistance using the same tests. In this study, we used a modification of the ASTM dry sand abrasion test (G65) that is under study by the ASTM G2 Committee for coating evaluation and a newly developed tape abrasion test to rank the relative abrasion resistance of common diffusion treatments (carburizing, nitriding, boronizing etc.) and compare their abrasion resistance with that of chromium, and electroless nickel platings. These same surface treatments were compared in a block-on-ring wear test (G77) self-mated and against a carburized counterface. The test results are intended to be a selection aid for use of these surface treatments in machine design.

Tool wear in cutting plastic : abrasion or erosion?

Abstract Tools used to cut, blank and perforate plastics do not seem to wear like one might expect. Cutting edges do not round and loose sharpness as do tools used to cut, blank and perforate metals. A die hole will develop a crater that abuts the periphery of the hole. The crater can have a depth of several micrometers and a width of about a millimeter. The same type of material removal pattern occurs on the end of flat-ended punches. The tool edges maintain sharpness because of this serendipitous material removal. What is producing this wear? Is it simple abrasion from hard phases in the plastic that is being punched or is it erosion from relative motion of the plastic product as it is compressed by the punch? This question was addressed in laboratory experiments conducted in a small punch press. A spring-loaded hemisphere (to simulate the punch) impacted a polished steel counterface (the die) with sheet plastic between the two. The spring-loaded hemisphere produced lateral movement of the plastic on the polished metal or cemented carbide counterface. Fresh plastic material is interspersed between the simulated punch and die for each press stroke. Damage to the counterface was assessed after 10 6 “punches” using profilometry and microscopy. The test results suggest that the predominating mechanism of punch and die wear in finishing cellulosic and thermoplastic polyester webs is erosion by relative motion of the product. The relative motion is caused by elastic deflection of the plastic as it is compressed. The product is behaving like a fluid and material removal is produced by mechanical interaction of a fluid with a solid surface, as required by the definition of erosion. It is proposed that this testing technique be used to assess the type of tool wear that can be anticipated from new plastic products.

Erosion of 316 stainless steel by dicalcium phosphate

Abstract A solids-handling facility for a chemical manufacturing organization was in the planning stage and a concern arose on whether special precautions should be taken to address erosion of stainless steel piping that was intended for use as the conveyance medium. The specific question at hand was: can reasonable service life ( > 10 years) be obtained with type 316 stainless steel for air conveyance piping of dicalcium phosphate at a velocity of less than about 50 m s−1? The fluence could be several kg s−1. The dicalcium phosphate is a by-product of gelatine manufacture and it is sold in dry-powder form as fertilizer and animal feed. The conveyance piping would be 100 mm in diameter and would involve significant length. If it was shown that the pipe would wear through quickly, steps could be taken to design the system with hard elbows or elbows with wear-backs to address particle erosion. Laboratory tests are described that indicated that the material is abrasive, but erosion damage would be minimal. The material forms a transfer film that mitigates erosion.

Control of fretting corrosion

Abstract The results of a laboratory study on the effectiveness of coatings and surface treatments in preventing fretting corrosion are presented.

Cemented carbide and cermet tooling for film-perforating

Abstract The tools used to perforate a particular photographic film started to wear at an unacceptable rate when the film base was changed from cellulose triacetate to polyester (PET). A laboratory investigation was initiated to screen candidate tool materials and identify ones with potential for 10 times life improvement over cemented carbide (WC/10% Co). The screening tests started with abrasion and corrosion tests on various grades of cemented carbide, cermets and selected ceramics. Concurrent production trials indicated that the laboratory corrosion tests were not correlating with production results. To address this problem, a “nibbler” test was developed which simulates perforating and material removal on a punch after 10 6 perforations (nibbles) became the screening test metric. It was determined that abrasion tests do not accurately predict tool material behavior when chemicals are present on or in the materials being perforated. Static corrosion tests do not predict tool response under production conditions. The rubbing of the film on the tool surfaces removes protective films and there can be a significant corrosion component in tool erosion. The nibbler simulates real tool conditions because erosion is produced by actual cutting of coated webs. Nibbler tests in this study indicated that alumina/zirconia resisted film erosion better than cemented carbide, even cemented carbide with PVD coatings. The nibbler tests also indicated that leaving recast layers from electrical discharge machining on cemented carbide greatly increases erosion rates. It should be removed. Production tests conducted since completion of these laboratory studies suggests that nibbler results correlate with production results. Coated cemented carbides are providing 3 times the service life of uncoated cemented carbides as predicted by the nibbler test.