Abstract: Low alloy steels for use in pressure vessel comprising on the weight % basis:C: from 0.05% to 0.30%,Si: less than 0.10%,Mn: from 0.3% to 1.5%,Ni: from inevitably incorporated content to 0.55%,Cr: from 1.5% to 5.5%,Mo: from 0.25% to 1.5%,V: in excess of 0.10% and less than 0.6%, andthe balance of iron and inevitably incorporated impurities.The steels are excellent in hardenability, the hot strength, toughness weldability and hydrogen attack and embrittlement resistance, as well as show excellent toughness after the use in the temper brittle temperature region.

Abstract: A method for detecting brazing defects in a plate-fin type heat exchanger with a brazed core structure having a number of wavy fin members alternately with a number of parallelly spaced plate members, in which a pulsed sound in the frequency range of several tens KHz to several hundreds KHz is transmitted across the fin and plate members from a pulser located on one outermost plate member to a sensor located on the other outermost plate member, detecting the positions and sizes of brazing defects two-dimensionally on a plane on the basis of the level of sound received by the sensor. Three-dimensional detection of the position of the defect is also possible by combining therewith a radiographic inspection using a radiant ray source which is located on one lateral side of the core structure to irradiate a radiant ray in a direction substantially parallel with the plate members of the core structure.