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A Machinability Rating Indicator for Tubes for Mechanical Applications Machinability, or useful tool life, is an important economic factor in machine tool operations. Tool life, surface finishing, chip removal and cutting forces are the principal factors to take into consideration when asking how machinable a material is. <strong>Focus</strong>ing specifically on tool life, determined by wear of the cutting edge, a Machinability Control Factor (MCF) has been developed by Emanuele Paravicini Bagliani, Product Engineer. It can be employed in order to quantify the advantages of modifying the production cycle, all the way from steel making to heat-treating the tubular product. Factors affecting machinability Usually in regards to material machinability, engineers adopt the Taylor equation which describes the link between tool life (L) and cutting speed (U): U · L α = C, α and C being two parameters which are experimentally determined. In order to do that, Taylor tests were conducted at several laboratories: I.V.F. (Institutet for 8 Tenaris News Verkstadstenkniks Forskning, Sweden), C.S.M. (Centro Sviluppo Materiali, Rome, Italy), CNR – ISTEC (Turin, Italy). Several steels with different microstructures and chemical composition were considered. In practice the optimal cutting speed is a balance between minimizing working time, idle time to change the insert and its relative cost. To evaluate the impact of machinability on production costs it is necessary to forecast the life of the insert in set cutting scenarios. Machinability as defined by the life of the insert depends on the microstructure, Knowing the chemical characteristics, microinclusions and microstructure of the steel enabled the MCF to be built. Tenaris has Developed a Machinability Control Factor to Forecast the Machinability of Steel. chemical composition, presence and distribution of secondary phase particles and mechanical properties of the steel being machined. Development of the MCF index To build an accurate forecasting model detailed microstructural and microinclusional measurements would have to be taken to build a correlation between production procedures and machinability. On the other hand, a simple indicator (MCF) using measurements easy to obtain can be used to forecast material machinability with sufficient accuracy based on the following parameters:
RELATIONSHIP BETWEEN MCF AND U10 U10 (m/min) 600 500 400 300 200 100 0 100 FIGURE 1 150 200 250 300 350 MCF 400 450 500 550 . . . Hardness Carbon content Sulfur content Ca/Al ratio With these four parameters, it’s possible to evaluate the microstructure and the presence of carbides as well as the types of inclusions. Tests were carried out on seamless tubes for mechanical applications in a range of dimensions in carbon and micro alloyed steel, completely killed and with a minimum guaranteed aluminum content, tested ‘as rolled’, normalized and quenched and tempered. In developing an index the cutting speed was set as corresponding to a cutting edge life of 10-minutes (U10). The conditions adopted are summarized in the table below. The objectives of the tests carried out were to determine: The material parameters (C, α) that link tool life (L) with cutting speed (U), as described in the Taylor equation mentioned earlier. A machinability index, also said the cutting speed which corresponded to a tool life of 10 minutes (U10). Obtaining the chemical characteristics, microinclusions and microstructure of MAIN FEATURES Reference standard UNI ISO 3685:1981 End of life criteria Maximum flank wear: VB=0.30 mm Maximum crater wear KT=0.18 mm Cutting operation dry turning Cutting insert Coromant CNMG 120408-PM 4015 Rake angle k=75° Cutting Depth 2.5 mm Feed 0.4 mm/rev 100° corner, clearance angle: -6° the steel enabled the MCF to be built. The relationship between the MCF and a cutting speed equal to a tool life of 10 minutes (U10) is illustrated in figure 1. As it can be observed the MCF represents a good machinability index. Conclusions With simple, easy to obtain measurements it is possible to gain good tool life prediction for the cutting conditions used in the test and quantify the effects of the factors analyzed here. Carbon content and hardness can hardly be used to control machinability because they are generally fixed by customer specifications, but understanding their effects can be important when planning the production cycle for a component, adding for example a heat treatment to change the hardness. On the other hand, sulfur content and the Ca/Al ratio can contribute to up to a 60- 70 m/min improvement in the (U10) value for a common steel for mechanical applications like 20MV6, which means improving the machinability index by as much as 10-15%. Based on experimental results conducted to answer the need for a product with superior characteristics in hydraulic cylinder, roller, bushing, connector and gear applications, Tenaris has developed special seamless tubes with high machinability (HL) and close tolerances for mechanical applications: TAM ® plus. Tenaris News 9
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