Strip Reduction Test Research Articles

Effect of workpiece pre-straining on tribological performance of surface coatings in sheet metal forming

The material condition of a workpiece changes as it is formed. Typically, the workpiece hardens and undergoes geometrical and topographical changes. In progressive forming, where workpieces are formed in multiple steps, this means that the workpiece in subsequent steps is different from the as-received material. In this work, strip reduction of an as-received strip and of a pre-strained strip are compared. EN1.4404 stainless steel is pre-strained by bending-under-tension. This leads to changes in the material condition and the surface topography of the strip. Two different coatings are tested in a strip-reduction test to evaluate the effect of these changes to the workpiece state on the tribological performance of the tool coatings. A coating that performed poorly when used to reduce the as-received material exhibited an improved performance when used to reduce the pre-strained material.

Prevalence of Quinolone Resistance Genes gyrA and gyrB in Bacteria Isolated from Urinary Tract Infections

Introduction: Antibioticshave significant importance in medicinal drugs but unfortunately bacteria can adapt various resistance mechanisms against those due to their abuse. A frequent way through which nitrate-reducing uropathogens acquire resistance mechanism against fluoroquinolones group of antibiotics is by continuously occurring mutation either in gyrA or gyrB genes through altering enzyme DNA gyrase that help in DNA replication. Microorganisms causing Urinary Tract Infection (UTI) vary in their susceptibility to antimicrobials from place to place and from time to time. Aim: The current study was performed to find the prevalence of nitrate-reducing bacteria in district Kohat and Hangu, their antimicrobial sensitivity pattern to various fluoroquinolones group of antibiotics and detection of mutation in gyrA and gyrB genes in the resistant bacterial isolates. Nitrate-reducing bacteria i.e E.coli, Klebsiella and Proteus are considered to be the frequent cause of UTI that reduce nitrate to nitrite by releasing enzyme nitrate reductase. Methodology: A Total of 252 urine specimens of suspected UTI patients were collected from Districts Head Quarter Hospitals (DHQ) and Combined Military Hospital (CMH) hospitals of districts Kohat and Hangu, Pakistan. Specimens reactive to nitrate strip reduction test were inoculated on (Cysteine Lactose Electrolyte Deficient) CLED media, and incubated at 37°C for 24 hours. Ciprofloxacin, levofloxacin, norfloxacin, ofloxacin and moxifloxacin fluoroquinolones group of antibiotics were used for to check their sensitivity patterns. Result: One hundred and seventy-five (69.44%) urine specimens were positive for nitrate reduction test, including 135 (77.14%) E. coli, 33 (18.85%) Klebsiella and 7 (4.0%) Proteus, verified through biochemical tests. Conclusion: Among the nitrate-reducing bacteria, fifty-one (37.22%) out of 135 E. coli, 9 (27.27 %) out of 33 Klebsiella and 2 (28.57%) out of 7 Proteus were resistant to most of fluoroquinolones group of antibiotics with moxifloxacin being the least resistant while ofloxacin being the most resistant antibiotic Malformed DNA gyrase producing mutated gyrA and gyrB genes were further detected in nitrate-reducing bacteria showing resistant to fluoroquinolones.

A Study on DLC Tool Coating for Deep Drawing and Ironing of Stainless Steel

The trend in metal forming tribology is to develop new tribo-systems including new lubricants, tool materials and tool coatings in order to substitute environmentally hazardous lubricants by environmentally friendly tribo-systems. In preliminary testing the limits of lubrication of new tribo-systems for sheet forming production, it is advantageous to use dedicated simulative tribo-tests. This paper studies the influence of tool coatings on deep drawing operations using the Bending Under Tension (BUT) test and also under more severe tribological conditions by adopting the Strip Reduction Test (SRT) to replicate industrial ironing of deep drawn, stainless steel parts. Non-hazardous tribo-systems in form of a double layer Diamond-like coated tool applied under dry condition or with an environmentally friendly lubricant were investigated via emulating industrial process conditions in laboratory tests. Experiments revealed that the double layer coating worked successfully, i.e. with no sign of galling, when it was used with environmentally friendly lubricants, whereas the results were more prone to galling under dry condition.

A study of DLC coatings for ironing of stainless steel

Stamping of sheet metal components without lubrication or using minimum amount of hazard free lubricant is a possible solution to diminish health hazards to personnel and environmental impact and to reduce production costs. This paper studies the application of diamond-like coating (DLC) under severe lubrication conditions by adopting strip reduction testing to replicate industrial ironing production of deep drawn, stainless steel cans. Three DLC coatings are investigated; multi-layer, double layer and single layer. Experiments revealed that the double layer coating worked successful, i.e. with no sign of galling using no lubrication even at elevated tool temperature, while the other two coatings peeled off and resulted in severe galling unless lubrication was applied.

The Influence of Tool Texture on Friction and Lubrication in Strip Reduction Testing

While texturing of workpiece surfaces to promote lubrication in metal forming has been applied for several decades, tool surface texturing is rather new. In the present paper, tool texturing is studied as a method to prevent galling. A strip reduction test was conducted with tools provided with shallow, longitudinal pockets oriented perpendicular to the sliding direction. The pockets had small angles to the workpiece surface and the distance between them were varied. The experiments reveal that the distance between pockets should be larger than the pocket width, thereby creating a topography similar to flat table mountains to avoid mechanical interlocking in the valleys; otherwise, an increase in drawing load and pick-up on the tools are observed. The textured tool surface lowers friction and improves lubrication performance, provided that the distance between pockets is 2–4 times larger than the pocket width. Larger drawing speed facilitates escape of the entrapped lubricant in the pockets. Testing with low-to-medium viscosity oils leads to a low sheet roughness on the plateaus, but also local workpiece material pick-up on the tool plateaus. Large lubricant viscosity results in higher sheet plateau roughness, but also prevents pick-up and galling.

Detection of the Onset of Galling in Strip Reduction Testing Using Acoustic Emission

Galling is an important issue in metal forming of tribologically severe materials such as high strength steel, stainless steel, Al- or Ti-alloys, since it leads to poor surface quality of the formed components, production stops and possibly deterioration of tools. The onset of galling is difficult to detect, since it is either based on the operator's personal judgement or indirect measuring techniques. The application of acoustic emission measuring technique for characterization of onset of galling in sheet metal forming is discussed in the presented paper. The strip reduction test, which emulates the ironing process, has been examined in order to evaluate onset of galling and how this is related to the generated acoustic emission parameters. Preliminary investigations have shown that differences can be found in the acoustic emission signal parameters depending on the frictional conditions between the tool and the workpiece surfaces. A correlation to the severity of galling is found. This is inspected through observations of tested workpiece surfaces in SEM and measurements of the surface roughness. The acoustic emission measuring technique is found to possess promising aspects for online monitoring of galling in metal forming processes.

Continuous Strip Reduction Test Simulating Tribological Conditions in Ironing

Abstract Laboratory testing of tribo-systems for sheet metal forming applications must ensure similar conditions with the tribo-parameters that are commonly utilized in real production in order to generate data that is meaningful for industry. The main parameters to consider are the tool and workpiece materials, surface roughnesses, normal pressure, sliding length, sliding speed, interface temperature and lubrication. This paper proposes a new Strip Reduction Test (SRT) for industrial ironing processes that is capable of replicating the highly severe tribological conditions that are experienced during both the forward stroke and the backward retraction of the punch. The new SRT tool design is implemented in a new Universal Sheet Tribo-Tester (UST), which can run multiple tests continuously from a coil. The test is capable of simulating various process parameters such as reduction, drawing speed, tool temperature, sliding length and quantifying the onset of breakdown of the lubricant film and subsequent galling after several strokes not only when emulating the forward strokes but also the backward strokes. Preliminary tests disclose promising results as regards the identification of lubricant film breakdown by detecting changes in measured force, surface roughness and/or torque values.

Influence of tool texture on friction and lubrication in strip reduction

Tool texturing is studied as a method to enhance lubrication and prevent the occurrence of galling. Strip reduction test tools manufactured with longitudinal, shallow pocket geometries oriented perpendicular to the sliding direction are tested. The pockets have small angles to the workpiece surface and varying distance. The experiments show an optimum distance between the pockets to exist that creates table mountain topography with flat plateaus and narrow pockets in between. If the flat plateaus are too narrow, an increase in drawing load and pick-up on the tool plateaus is observed. The same occurs for too wide plateaus. A theoretical friction model supports the experimental findings of an optimum distance between the pockets, where the contribution to friction by mechanical interlocking of the strip in the pockets is limited and lubrication of the plateaus is enhanced by micro-plasto-hydrodynamic lubrication.

Testing and modelling of new tribo-systems for industrial sheet forming of stainless steels

Sheet metal forming of stainless steels is known to be tribologically demanding. To ensure satisfactory production without pick-up and galling, lubrication with environmentally hazardous chlorinated paraffin oil is normally required and in the most severe cases combined with ceramic tool coatings. The present paper presents an attempt to clarify the challenges of replacing chlorinated paraffin oils with new environmentally friendly oils in an industrial production, by investigating these oils in combination with alternative tool materials and tool coatings. A number of tribology systems have been screened in the laboratory by strip reduction testing and the best tribo-systems have been tested in a progressive production tool at the Danish company Grundfos A/S. The combination of a TiAlN coating with one of the environmentally friendly lubricants shows good performance in the simulative test as well as the production test with no sign of galling even at maximum production speed. Lowering production speed, and thereby the interface temperature, allows less efficient tribo-systems to function satisfactory, i.e. without galling. The tool/workpiece interface temperature is simulated for the strip reduction test as well as the production test in order to estimate the critical interface temperature for lubricant film breakdown. Simulation results show good agreement with experimental measurements of tool temperature close to the interface.

Prediction of limits of lubrication in strip reduction testing

Pick-up and galling due to lubricant film breakdown is a severe limitation in cold forming of tribologically difficult metals like stainless steel and aluminium. The present paper describes a method of combined experimental and numerical analysis to quantify the limits of lubrication in a dedicated simulative strip reduction test. The limit of lubrication is quantified as the threshold drawing length before galling occurs. A numerical model of the test is established calculating tool/work piece interface temperatures and normal pressures. Identifying a critical maximum value of the interface temperature the results show good agreement between numerically predicted and experimentally observed threshold drawing lengths at different test conditions.

On the chemical nature of boundary lubrication of stainless steel by chlorine- and sulfur-containing EP-additives

The nature of the extreme pressure (EP) effect of the dialkylpolysulfides and chlorinated paraffins during the ironing of stainless steel AISI 304 has been studied. A strip reduction test was used in combination with differential thermal analysis (DTA), profilometry, X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy analysis (AES). Strip reduction tests, in which chlorinated paraffin and dialkyl sulfide were compared as EP-additives, have shown significantly stronger EP-effect of the chlorine containing lubricant, than for the sulfur containing lubricant. DTA analysis of the Me-powder (where Me=iron, chromium or nickel)–dialkylpolysulfide (or chlorinated paraffin) mixtures have shown that dialkylpolysulfide was chemically active with iron and nickel (the highest activity was with nickel). Chromium was practically nonactive with dialkylpolysulfide. Chlorinated paraffin was equally active with iron, chromium and nickel. The better lubrication performance demonstrated by chlorinated paraffin compared to dialkylpolysulfides was attributed to the chemical activity of the chlorinated paraffin with all the main components of stainless steel. The depth profiles of the stainless steel strips were examined before and after strip reduction by use of AES sputter profiling. Results imply that the chlorine containing lubricant enforces the formation of a thick oxide layer.

Quantification of galling in sheet metal forming by surface topography characterisation

One of the major problems in forming of stainless steel sheet is galling due to lubricant film breakdown leading to scoring and bad surface quality. In a Danish research programme new lubricants substituting the normally applied chlorinated paraffin oils are being developed and tested for this purpose. In order to determine the limits of lubrication of these new lubricants, as well as commercial ones already available on the market, two sheet forming tests have been developed. Quantification of the degree of galling is done by roughness measurements on the workpiece surface. In a strip reduction test, this is done by 2D profilometry at strip locations corresponding to different sliding lengths, whereas a deep drawing test is based on 3D roughness mapping of local areas.

Screening the performance of lubricants for the ironing of stainless steel with a strip reduction test

A laboratory strip reduction test simulating the tribological conditions of an ironing process is proposed. The test is capable of simulating varying process conditions such as reduction, drawing speed, tool temperature and sliding length. The test makes it possible to quantify the onset of breakdown of the lubricant film and subsequent galling. Experimental investigations of stainless steel show the influence of varying process conditions and the performance of different lubricants.