A steel pipe for pressure piping subjected to autofrettage has an average hardness at its outer layer region of 1.20 times or more of an average hardness at its inner layer region. When an outer diameter is D, and an inner diameter is d, a measured value of a residual stress at an outer surface is denoted by ?.sub.o1, a measured value of a residual stress at an outer surface after halving is denoted by ?.sub.o2, and a measured value of a residual stress at an inner surface after the halving is denoted by ?.sub.i2, an estimated value ?.sub.i1 of a residual stress at the inner surface of the steel pipe is determined by [?.sub.i1=(??.sub.i2)/(A?(t/T).sup.2?1)], [t/T=((?.sub.o2??.sub.o1)/(A?(?.sub.o2??.sub.o1)?C??.sub.i2)).sup.1/2], [A=3.9829? exp(0.1071?(D/d).sup.2)], and [C=?3.3966?exp(0.0452?(D/d).sup.2)] is ?150 MPa or less.

A steel material according to the present disclosure has a chemical composition consisting of, in mass %: C: 0.15 to 0.45%, Si: 0.05 to 1.00%, Mn: 0.01 to 1.00%, P: 0.030% or less, S: 0.0050% or less, Al: 0.005 to 0.100%, Cr 0.55 to 1.10%, Mo: 0.70 to 1.00%, Ti: 0.002 to 0.020%, V: 0.05 to 0.30%, Nb: 0.002 to 0.100%, B: 0.0005 to 0.0040%, N: 0.0100% or less, O: less than 0.0020%, and the balance being Fe and impurities, and satisfying Formula (1) described in the specification. A grain diameter of a prior-austenite grain is 15.0 m or less, and an average area of precipitate which is precipitated in a prior-austenite grain boundary is 12.510.sup.3 m.sup.2 or less. A yield strength is 758 to 862 MPa.

A method for the autofrettage of a workpiece may involve arranging the workpiece between a first securing means and a second securing means, and applying high-pressure fluid to an internal volume of the workpiece that is formed between the first and second securing means. A die or ram may be driven into the internal volume through an inlet in the first securing means. Consequently, as a result of the ram being advanced, not only is a fluid pressure generated in the internal volume, but also the workpiece is mechanically autofrettaged by way of the ram. A device may be employed to perform such hydromechanical autofrettage of the workpiece.

A method for the autofrettage of a workpiece may involve arranging the workpiece between a first securing means and a second securing means, and applying high-pressure fluid to an internal volume of the workpiece that is formed between the first and second securing means. A die or ram may be driven into the internal volume through an inlet in the first securing means. Consequently, as a result of the ram being advanced, not only is a fluid pressure generated in the internal volume, but also the workpiece is mechanically autofrettaged by way of the ram. A device may be employed to perform such hydromechanical autofrettage of the workpiece.

An expansion tool comprises a tool body for manipulating the tool, a drive module for driving the rotation of a cylindrical needle cage in the bore, the cylindrical needle cage having an axis of revolution about which the needle cage is configured to start to rotate in the bore, the needle cage comprising needles configured to move radially away from the axis of revolution so as to work-harden the bore as the needle cage is rotationally driven by the drive module. The expansion tool also comprises a rod having a burnisher configured to move the needles of the needle cage radially as the burnisher passes through the needle cage as a result of the translational movement of the rod. The rotation of the needle cage allows the entire internal surface of the bore to be work-hardened.

A method for increasing the fatigue strength of a tubular semifinished product for a medical implant, such as a stent or a cardiac valve, which includes providing a tubular semifinished product formed from a bioresorbable magnesium alloy, iron, or an iron-based alloy; and applying pressure at a temperature equal to, or less than, 420 C., internally against an inner surface of the tubular semifinished product such that the outer circumference of the tubular semifinished product is subject to plastic deformation by at least 0.2%.

A method for increasing the fatigue strength of a tubular semifinished product for a medical implant, such as a stent or a cardiac valve, which includes providing a tubular semifinished product formed from a bioresorbable magnesium alloy, iron, or an iron-based alloy; and applying pressure at a temperature equal to, or less than, 420 C., internally against an inner surface of the tubular semifinished product such that the outer circumference of the tubular semifinished product is subject to plastic deformation by at least 0.2%.

A steel pipe or tube for pressure vessels having excellent quench crack resistance is provided. The steel pipe or tube for pressure vessels comprises: a specific chemical composition; and a metallic microstructure in which an average grain size of prior austenite grains is 500 m or less, and an area fraction of microstructures other than ferrite is 50% or more.

A method for increasing the fatigue strength of a tubular semifinished product for a medical implant, a tubular semifinished product for a medical implant having improved fatigue strength, and a medical implant produced from such a semifinished product.

A method for increasing the fatigue strength of a tubular semifinished product for a medical implant, a tubular semifinished product for a medical implant having improved fatigue strength, and a medical implant produced from such a semifinished product.