A specification for steel forgings such as ASTM A694 covers a wide range of strength grades from F42, with a minimum specified yield strength of 290 MPa, to F70, with a minimum specified yield strength of 485 MPa. The compositional limits are given in ASTM A694 (product analysis) are ≤0.3% carbon 0.13-0.37% silicon and ≤1.50% manganese. Additionally, the steel may contain micro-alloying elements, i.e. it may be a micro-alloyed (or high strength low alloy) steel; alternatively, Cr-Ni-Mo-V low alloy steel, ASTM A707 Grade L5, may be used.
Particularly for small valves and thin section sizes, it may be expected that the strength and impact toughness requirements of the lower strength grades can be met with carbon-manganese steel. However, if the use of the low alloy steel grade is to be avoided, it will probably be necessary to use micro-alloyed steel in order to meet the strength and toughness requirements of the higher strength grades. It is likely that the steel will have been subjected to more than one cycle of heating to temperatures of at least 900°C, prior to hot forging operations. Whilst a simple normalizing treatment should then be sufficient to refine the grain size and to restore mechanical properties to a sufficient level to meet the tensile requirements for the lowest strength grades, particularly in thin sections, such a treatment may be inadequate for higher strength grades, including F65, particularly at large section thicknesses, in the larger diameter valves.
It should be recognized that the presence of vanadium nitride or niobium carbo-nitride precipitates restricts grain growth during a normalizing treatment. However, with prolonged heating at the forging temperature, growth of the nitride or carbo-nitride precipitates will occur, and their distribution may become such that they are no longer capable of restricting austenite grain growth adequately. Thus, the resulting ferrite grain size on cooling may be larger than the optimum grain size. As a consequence, after a subsequent normalizing treatment, both strength and toughness may be below the maximum achievable levels. The mechanical properties can, however, be restored by solution treating at a sufficiently high temperature that the vanadium nitride or niobium carbo-nitride precipitates are taken back into solution. According to Fig.1, this requires a temperature of at least 1050°C for vanadium nitride or 1200°C for niobium carbo-nitride. The resulting prior austenite grain size will, however, be coarse, and so a subsequent normalizing treatment, at a temperature of typically 900°C, will be required.
Whilst it is industry experience that such a double heat treatment, when required, is sufficient to restore mechanical properties for ASTM A694 F65 thick-section forgings, the standard does not draw attention firstly to the need to choose a micro-alloyed steel, and secondly to the fact that a high-temperature solution treatment, and subsequent normalizing, may be required in order to meet the mechanical property requirements of the specification. Thus, a valve manufacturer, even if it chooses to use micro-alloyed steel, may fail to meet the mechanical property requirements of the higher strength grades at thick section sizes, if the manufacturer’s sole guide, with regard to heat treatment, is section 5.3 of ASTM A694.
The flanges and fittings and the bulk-head specifications similarly do not give detailed guidance on solution heat treatment, but rather rely on the experience of the manufacturer, as they require the manufacturer to submit a detailed heat treatment procedure, indicating how specified mechanical properties are to be achieved.
As C-Mn steel weld metals would be softened substantially by such a double heat treatment, any welding should be carried out after such heat treatment, or welding consumables that will withstand normalizing should be used. If valves or fittings containing C-Mn steel weld metal have been so heated, the weld metal should be removed and the joints re-welded.
Fig.1. The times of complete dissolution of various carbides and nitrides in austenite as a function of temperature, after K. Easterling, ‘Introduction to the Physical Metallurgy of Welding’, Butterworth/Heinemann, Second Edition 1992