Study of deformation conditions at longitudinal pipes rolling from austenite steel grades

Stainless steels of austenite class 08–12Х18Н10Т have a high corrosion resistance, which stipulates for their wide application in various areas of industry. Technology of pipes production of the steels is rather specific and requires observation of some conditions. It was shown that temperature of a work-piece heating before deformation is an important parameter of the technology. It was noted that for the piercing of a steel work-piece with various chrome content, there is a rational temperature interval. Nonobservation of the temperature can lead to defects formation on internal pipe surface because of earlier destruction and opening of metal cavity during piercing. The choice of the rolling-out scheme has a direct effect on the work-piece forming in cross-sections. Results of hot rolling of Ø37×2,5 tube samples, manufactured of 08–12Х18Н10Т steel and carbon steel of grade 40 presented. The rolling was done at a laboratory mill. As a result of the experiment the lower limit of ovality of rolls grooves was specified for conditions of rolling of pipes from 08–12Х18Н10Т steels by 2-roll scheme. At the rolling with ovality B/H ≤1,07, defects appeared on the internal tube surface in the form of scratches caused by the mandrel. The rational range of ovality of grooves at multi-stand rolling can be from 1.08 to 1.15. According to criterion of groove overfilling by metal for steels 08–12Х18Н10Т, requirements were formed towards the groove width of the first stand of longitudinal rolling mill. The groove width must be larger than the sleeve diameter: for 3-roll scheme – at least by 2–3%, for 2-roll scheme – at least by 7.0–7.5%. Potential advantages of 3-roll scheme comparing with 2-roll scheme for rolling of 08–12Х18Н10Т steels were established as follows: lower probability of grooves overfilling by metal of the work-piece, absence of defects (scratches caused by mandrel), on the tube internal surface at minimal ovality level of 1.07, lower level of transverse pipe wall non-uniform thickness.

COMPREHENSIVE STUDY OF THE DYNAMICS OF THE RETAINING MECHANISM OF THE AUTOMATIC MILL

The results of the study of the dynamics of the mechanism for holding the mandrel of an automatic TPA mill are presented. The value of the axial force acting on the mandrel and the rod system of the mechanism of its holding is determined. Developed models of the mandrel retention mechanism have been developed, which made it possible to establish the nature of the development of dynamic processes on the output side of the automatic mill, taking into account the force parameters of the longitudinal rolling process of the sleeve and the parameters of the output side of the automatic mill. Mathematical models of the mandrel retention mechanism have been developed, taking into account the parameters of the technological process and the inertness of the rolled sleeve. The solution of the system of differential equations with variable coefficients is implemented numerically using the Runge-Kutta method for the corresponding dynamic models of the mandrel retention mechanism on the example of calculating the automatic mill TPA 350. The picture of the formation of dynamic processes in the mandrel retention mechanism during the implementation of the entire technological process of pipe production on an automatic mill has been clarified. taking into account the variable impact from the deformation zone and the change in the mass of the rolled sleeve. For various dynamic models, numerical solutions of problems were implemented, which made it possible to select the necessary technological and dynamic parameters of the mechanical system, to assign stable modes of rolling the sleeves on the automatic mill TPA 350. It is shown that the refined solution of the problem for three developed dynamic models of the mechanical system differs significantly from the previously known mathematical models. It has been established that the dynamics of the mechanism for holding the mandrel of the automatic TPA 350 mill is equivalent to the formation of the mechanism for the formation of different wall thickness of pipes. A mechanism for stabilizing the dynamics of the mandrel holding mechanism by separating the sections of the output side of the mill is proposed.

Study of the process of formation of tubes end sections during longitudinal rolling in grooves formed by a different number of rolls

The work substantiates the relevance and highlights the degree of development of the research topic. The main advantages and disadvantages of the process of rolling hollow billets with the use of modern longitudinal rolling mills with 2 and 3 roll grooves are noted. With the help of physical and computer modeling, a study of the process of longitudinal rolling of pipes using 2 and 3 roll passes was carried out, for the formation of end sections (scrap). Established and numerically reflected the relationship between the size of the reduction of passes of a continuous rolling mill with 3-roll and 2-roll stands and the length of the end sections of the pipes. It is clear that the use of 3 rolls, forming a groove, allows to reduce the total end cut by 1.5 times, in contrast to 2 rolls. It has been determined that the main technological factors influencing the formation of the end sections are the ratio of the external diameter to the wall thickness of the strip (D/S), the reduction along the wall thickness, the elongation ratio, and the number of rolls required to form the groove. An empirical formula has been obtained for calculating the length of the end trim, which makes it possible to evaluate the influence of the number of rolls in the stand, the D/S ratio and the stretch ratio.

Surface Purity Effect on Irregularities of Changes in Deformation Texture of Zr-2.5% Nb Alloy

This work is a continuation of a series of works on the study of regularities and structural mechanisms of changes in characteristics of crystallographic texture during cold deformation of plates made of Zr2.5%Nb alloy. Effects of influence of surface cleanliness of the plates on the textural regularities during their rolling were investigated. For this, longitudinal fragments of the tube Æ15.0´1.5 mm² were used, flattened, annealed at 580°C in a vacuum of 1.5...3.0 Pa and rolled along the axis of the original tube with various degrees deformation up to 56%, which is likened to longitudinal rolling of plates. Techniques of maximally uniform straightening of tube fragments were used. An analysis of the results of studies of textural changes during cross rolling of plates, straightened from rings of the same tube and pretreated under similar conditions, is also carried out. To analyze the results, the method of inverse pole figures was used, which, in these studies, is distinguished by the possibility of achieving satisfactory accuracy in calculating the integral characteristics of texture. On this basis, the Kearns textural coefficient was calculated along the normal to the plates’ plane. Corrections were introduced for texture dissimilarity along the thickness of the plates, which is caused by the unbending of the preliminary blanks. Additionally, the analysis of texture distributions was carried out using original techniques. According to the results obtained – as a result of X-ray measuring from the plates’ surface – oscillations of the course of changes in the texture coefficient were revealed. This is associated with an alternating process of relaxation of residual stresses during deformation. It has been established that this effect is initiated from the near-surface regions, is associated with a near-surface impurity, and in some cases can penetrate to a considerable depth of the plates. The twinning nature of such regularities is confirmed and active systems of twins are noted.

Dealing with Uncertainties in Fatigue Strength Using Deep Rolling

Mechanical properties inherently possess uncertainties. Among these properties, fatigue behavior data generally shows significant scatter which introduces a challenge in the safe design of dynamically loaded components. These uncertainties in fatigue behavior are mainly results of factors related to surface state including: Roughness, tensile residual stresses, scratches and notches at surface. Therefore, controlling these parameters allows one to increase fatigue strength and reduce scatter and uncertainties in fatigue behavior. Mechanical surface treatments are applied on parts to increase fatigue strength via introducing compressive residual stresses and work-hardening at surface. Two of the most common among these treatments are shot peening and deep rolling. Shot peening has found many applications in industry because of its flexibility. However, it introduces irregularities at the surface and may increase roughness which causes uncertainties in the fatigue behavior data; especially for low-medium strength materials. Unlike shot peening, deep rolling reduces surface roughness. Therefore, it has the capability to reduce uncertainty in the fatigue behavior. To this date, rolling direction of deep rolling was selected as tangential direction to turning direction for axisymmetric parts. Nonetheless, the authors believe that the rolling direction has an apparent effect on the fatigue behavior. In this study, longitudinal direction was also applied for deep rolling operation and the results of these two direction applications on the EN-AW-6082 aluminum alloy were investigated. It was shown that, longitudinal rolling had yielded less scatter and uncertainty in the fatigue behavior than the tangential rolling together with the higher fatigue strength.

ACCURACY WALL SICKNESS OF HOT-DEFORMED PIPES STATISTICAL ANALISIS

Pipe-rolling units with an automatic mill are a aggregate production complex that produces hot-rolled pipes of various sizes. The technological process of seamless pipes production, has many stages: flashing the billet into a sleeve, longitudinal rolling in gauge, running in a oblique rolling mill, calibrating and reducing the diameter. Each stage significantly affects the accuracy of the geometric dimensions of the pipes. One of the main parameters characterizing the accuracy of the pipes is their transverse difference namely the size and the nature of the distribution of the pipe wall thickness in the cross section. A significant reserve for saving metal is increasing the accuracy of seamless pipes to avoid different pipe wall thickness. Different pipe wall thickness makes it difficult to get quality pipe screw-thread.The conditions of metal deformation at an injection molding machine with an automatic machine do not exclude the presence of fluctuation of the pipe wall thickness. The best characteristic of the accuracy of finished pipes will be the knowledge of quantitative indicators of the transverse difference of their end sections. The purpose of this work is to determine the variation in wall thickness of hot-rolled casing by means of mathematical statistics methods.The use of statistical data processing methods makes it possible to predict the seamless pipe difference indicator. A statistical analysis of the wall thickness indicator of the end sections showed a high ratio of wall thickness symmetry. An effective way to minimize the symmetric difference component is to optimize the deformation modes along the pipe wall. In this case, a graphical analysis of the distribution of wall thickness showed that the actual difference varies in stochastic dependence. To clarify the general form of the random periodic component of such a dependence, it is advisable to apply methods of harmonic analysis, which will allow us to develop a mathematical model for determining the accuracy of pipes.