Papers by Keyword: Seamless Tubes

Abstract: Experimental research on the process of cold tubes drawing was performed in order to obtain important information about the behaviour of the tube material in the drawing tool during the drawing to the required outer diameter of Ø12 mm. The DEFORM-3D simulation software was an important tool for verifying the plastic flow of material in the drawing tool (i.e. drawing die), as well as for optimizing the technological parameters of cold tubes drawing. In the finite element (FE) analysis, emphasis was placed on determining the influence of tool geometry, strain degree, drawing speed and friction on the size of the drawing force and the quality of production of drawn tubes. The paper points out the importance of simulation software for common comparison of results obtained from laboratory experiment and FE analysis of the process of tubes drawing. By comparing the results, the reliability of the used simulation software was proved, as the results were identical for all monitored technological parameters. Computer simulation brings valuable knowledge that is important for the correct design of the drawing tool as well as for the technology of production of tubes with precise dimensions.

Abstract: By use of physical simulations, it was studied the influence of finish rolling temperature (from 820 °C to 970 °C) on the microstructural and mechanical properties of seamless tubes with a different wall thickness (from 6.3 to 40 mm) – in the state after rolling as well as after quenching and tempering. In laboratory conditions, by use of the Simulator HDS-20, the bloom piercing and rolling of the seamless tubes from 25CrMo4 low-alloy steel in a pilger mill were in a simplified way simulated. The wall thickness of the tube influenced the total deformation of specimens at anisothermal multi-pass plain-strain compression tests as well as the final cooling rate. The quenching and tempering of the deformed specimens was subsequently performed with use of the electric resistance furnaces. The finish rolling temperature had only insignificant effect on the resulting properties. Markedly lower hardness was obtained only after the simulation of tube production with the wall thickness of 40 mm contrary to the wall thickness of 6.3 and 20 mm. Structural variations of the specimens after rolling simulations were more or less overlapped by the subsequent quenching from the temperature of 850 °C and tempering at the temperature of 680 °C.

Abstract: Using the laboratory rolling mill with smooth rolls, piercing, as well as rolling in a pilger mill of the seamless tubes with diameter 273 mm from the HSLA steel microalloyed with vanadium steel was simulated. Influence of the wall thickness (6.3 – 40 mm) and finish rolling temperature on the final structural and mechanical properties was investigated. Necessary temperatures of the phase transformations in the course of cooling were determined by dilatometric tests. Based on the dilatometry results, finish rolling temperatures were reduced. Lower rolling temperatures yielded in a relative grain refinement. Effect of the finish rolling temperature did not have any marked impact on the tensile tests results. Strength properties decreased only slightly with the increasing wall thickness and the plastic properties were not influenced significantly by this parameter. The positive effect of the reduced finishing temperature appeared markedly in the results of impact tests performed at room temperature only. Notch toughness was increased by approx. 25 % in the case of the wall thickness of not less than 20 mm.

Abstract: Increased efficiency and emission reduction in modern power plants lead to the use of new advanced materials with enhanced creep strength, with the objective to increase the steam parameters of power plants. With over ten years on market and wide experience related to its use, ASTM Grade 92 is becoming one of the most required materials when high service temperatures are reached (max. 610°C). Its composition, with 9%Cr and 1.5%W, gives rise to martensitic microstructures which offer very high creep strength and long term stability. The improved weldability and creep-strength between 500 and 580°C of the low alloy ASTM Grade 23, as well as a cost advantage over higher Cr materials in this temperature range, make it one of the possible candidates to meet the stringent requirements of modern power plants. Air Liquide Welding (ALW) has optimized and distributes a complete product family for the welding of Grades 23 and 92. TenarisDalmine (TD) focused on the development of Grade 23 tubes and pipes and is working on the development of Grade 92. A deep characterization work of the microstructural evolution and long term creep performances of these high temperature resistant materials was thus undertaken by ALW and TD, in collaboration with the Centro Sviluppo Materiali (CSM). The joint characterization program consisted in the assessment of welded joints creep properties. Welded joints were produced using the gas tungsten (GTAW), shielded metal (SMAW) and submerged arc welding (SAW) processes. Mechanical and creep properties of weldments were measured both in the as welded and post weld heat treated conditions and proper WPS’s were designed in a manner such that industrial production needs were satisfied. Short term creep resistance of cross weld specimens was measured to be within the base material acceptance criteria. Long term base material and cross weld creep performance evaluation are now in progress.

Abstract: Seamless tubes are used for many applications, e.g. in heating, transport gases and fluids, evaporators as well as medical use and as intermediate products for hydroforming and various mechanical applications, where the final dimensions normally are given by some cold drawing steps. The first process step – piercing of the billet, for example by extrusion or 3-roll-milling - typically results in ovality and eccentricity in the tube causing non-symmetric material flow during the cold drawing process, i.e. inhomogeneous deformation. Because of this non-axisymmetric deformation and of deviations over tube length caused by moving tools, this process step generates residual stresses. To understand the interconnections between the geometrical changes in the tubes and the residual stresses, the residual strains in a copper tube had been measured by neutron diffraction.

Abstract: Hot rolling of seamless tubes is performed in two stages: one at high temperature, ie, above 1000oC and a second part at low temperature range, namely below 900oC. Above 1000oC, dynamic, metadynamic and static recrystallization may happen leading to full softening and grain refinement. Below 900oC, however, some pancaking may occur and ferrite is thus refined during transformation. Nonetheless, in order to refine ferrite grains further, cooling to room temperature and re-austenitization at an intermediate reheating furnace can be a viable alternative to be explored. The present paper examines the implications of re-austenitization followed by phase transformation plus low temperature deformation on ferrite grain refinement. Phase transformation on re-heating and cooling as well as recrystallization play important roles in the process of ferrite grain refinement. These mechanisms are analyzed and discussed. Results indicate that ferrite grain refinement is most effective in the case of processing using a re-austenitization cycle in the intermediate furnace followed by rapid cooling after deformation in the stretch reducing mill.

Abstract: The hot workability of modified 9Cr-1Mo, as a grade of heat resistant steels, is inferior to that of low-alloy steel, so the inner surface crack (ISC) easily occurs in seamless boiler tubes produced by the Mandrel Mill under improper rolling conditions. With the aid of FEM, the metal flow status during the rolling process was analyzed in 140mm 8-stand mandrel mill of Bao Steel. Both the metallographic shape and size of the ISC together with the result from the simulation show that the ISC of seamless tube forms at the elongation stage of shell. The mechanism of the ISC was discussed. With its initiation in stand No.1 and No.2 due to poor hot workability of modified 9Cr-1Mo steel, the ISC develops in subsequent passes. Based upon the mechanism devised was a special roll pass system which substantially upgraded the yield of qualified products.