Part 1: The Strain Aging Behavior of Microalloyed Steels

The strain aging behavior of five microalloyed steels was studied using a variety of straining, aging and stress relieving cycles. The materials studied were pressure vessel steels ASTM A737 Grade B and A737 Grade C, and structural steels ASTM A588 Grade A, A588 Grade B and ASTM A572 Grade 50, Type 2. The A737 Grade B and Grade C steels were tested in the normalized condition while the A588 Grades A and B and the A572 Grade 50 Type 2 steel were tested in both the as-rolled and normalized condition. The steels were cold strained in tension 2, 5 or 10% and were aged at 260 or 370°C (500 or 700°F) for 10 hr. Strained and aged specimens were stress relieved for 2 and 10 hr at 620°C (1150°F). Transverse mechanical properties were measured for the as treated, strained, aged and stress relieved steels.

The results of the study showed that all of the steels were sensitive to strain aging by increases in strength and losses in toughness. The average increase in 34J (25 ft-lb) Charpy impact transition temperature for 5 to 10% cold strain was 38°C (68°F). Post-strain stress relief at 620°C (1150°F) did not restore toughness to its original level, only reducing the transition temperatures by an average of 5°C (8°-F). Because of the transverse orientation tests, the strain-aged transition temperatures for the structural grades, A588 Grades A and B and A572 Grade 50 were equal to or above ambient temperature. Yield and tensile strength increased with straining and aging. Stress relief did reduce the strength again but not to its original values.

The extent of strain aging was sensitive to the amount of prior strain but not to specimen orientation to straining or aging temperature. Extended stress relief cycles did not result in any improvement in toughness. Aging alone did not result in a significant increase in transition temperature but stress relief treatments alone did for some materials.

Extensive shifts in transition temperature were produced by strain aging even after stress relief and this must be taken into account whenever the cold forming of these materials is considered, especially in the structural grades.

Part 2: The Fracture Toughness Behavior of ASTM A737 Grade B and Grade C Microalloyed Pressure Vessel Steels

The materials tested in this program are considered microalloyed steels, a class of low-alloy high-strength steels that utilizes relatively low carbon content and finely dispersed carbides to provide strength and toughness. This type of steel has been used for piping and structural applications under several specifications and has been incorporated into ASTM pressure vessel plate specifications such as A734, A735 and A737. The yield strength levels of these steels exceed those of normal carbon-manganese steels (A515 or A516) by 24% to over 100% (depending on grade). Their lowest tensile strengths are the same as the highest strength carbon steel grades and may be 21% higher. In addition, the Charpy impact transition temperature levels the steels are able to provide are much lower than conventional steels, which makes them superior in applications requiring low temperature toughness. Due to their generally low carbon content, their weldability is also good.

Because of these general characteristics, and to explore further their potential in the pressure vessel field, the Materials Division of the Pressure Vessel Research Committee, through its Pressure Vessel Steels Subcommittee instituted a study of selected steels of this type. The interest of the PVRC was to document their strength and toughness in the variations to be discussed below, and to do so in section thicknesses great enough to include most pressure vessel service. This paper is a report of the PVRC study.

Part 3: The Fracture Behavior of ASTM A737 Grade B and Grade C Microalloyed Steel Weldments

The strength and fracture toughness of weldments of A737 Grades B and C steel were determined over a range of temperatures both in the as-welded condition and after post-weld heat treatment. The two base metals were submerged arc welded at a heat input of 2.8 kJ/mm (70 kJ/in.) using Armco W-19 (3.5 Ni) filler metal and Linde 709-5 flux. The base metals were 76 mm (3.0 in.) (A737 Grade C) and 102 mm (4.0 in.) (A737 Grade B) in thickness and a "K" configuration multipass weld joint design was used to provide a relatively straight heat-affected zone for toughness testing. Post weld heat treatment at 593°C (1100°F) for 0, 2 and 10 hr was used as a test variable.

Part 4: Long Time Stress Relief Effects In ASTM A737 Grade B and Grade C Microalloyed Steels

An experimental program was conducted to investigate the effect of stress relief heat treatment at 620°C (1150°F) on the mechanical properties of a carbon-manganese-niobium steel (ASTM A737 Grade B) and a carbon-manganesevanadium-nitrogen steel (A737 Grade C) in two conditions of heat treatment, normalized, and quenched and tempered. Only modest changes in toughness occurred for stress relief treatment times of 10 hr or less at 620°C. For longer times, 30 hr or more, stress relief embrittlement was observed in the normalized steels, the extent being time and temperature dependent. The embrittlement was characterized by (1) non C-curve behavior, (2) no intergranular cleavage fracture, (3) no consistent effect of prior heat treatment, (4) no trough in toughness, and (5) no secondary hardening. From these observations the conclusion was made that neither classical temper embrittlement nor coherent carbide formation is responsible for the embrittlement observed in the A737 steels. A good correlation between toughness and the carbide thickness was found. From all of these results, the embrittlement mechanism was concluded to be carbide coarsening.

Strength changes were small for stress relief treatments of up to 10 hr at 620°C (1150°F) for the A737 Grade C, but were observed after 1 hr for the A737 Grade B. There were distinct decreases in yield and tensile strength for stress relief times that extended to 100 hr and beyond.