AISI 4340

Low alloy steel typically containing 0.3% Carbon and alloyed with 1% Chromium and 0.2% Molybdenum to give enhanced mechanical properties.
The grade has poor through hardenability with a typical ruling section of around 4 inches (at 75 Ksi) depending on the chemical composition, and it is critical that the hardenability of the grade must be taken into account when designing and selecting it forspecific equipment.
Impact toughness is generally good to temperatures as low as -46 Deg C with typically 27J average and 20J single achieved, this is limited though dependent on a number of factors such as ruling section, chemical composition and heat treatment condition, with impact toughness achieved decreasing with higher strength, increased ruling section and at lower test temperatures due to the materials Ductile / Brittle transition temperature.
Minimum hardness is based on API 6A requirements. Maximum hardness shown is based on compliance with NACE MR0175 / ISO 15156
Grade is typically used for pressure containing applications such as valve bodies or pipework, also used for valve gates and stems.
Very good weldability, and so may be readily welded to itself or weld inlayed. However there have been recent concerns on using the grade for buttered connections. The grade does require a post weld heat treatment at 620o C min in order to meet sour service (NACE MR0175/ ISO 15156) requirements

Equivalent/Similar Standards