SAE 4140

What is SAE 4140?

SAE 4140 is a chromium-molybdenum alloy steel containing 0.38 to 0.43 percent carbon. It is used in the quenched and tempered condition and is the most widely specified alloy engineering steel in the world. Its composition includes roughly one percent chromium and one-fifth of a percent molybdenum. The chromium allows a section to harden through during an oil quench. The molybdenum delays embrittlement and maintains a useful tempering range.

SAE 4140 and EN 19 are near-equivalent grades, and substitution between them is routine. They remain separate specifications. EN 19 has a higher minimum molybdenum content and a wider carbon range. SAE 4140 has tighter limits for sulphur and phosphorus. The American grade is purchased against chemical analysis, while the British grade is purchased against a mechanical property condition. Compliance should therefore be confirmed from the certificate before substitution.

European designations for the steel include 42CrMo4 and material number 1.7225. Its UNS designation is G41400.

Laxcon Steels lists SAE 4140 in its grades reference as an alloy steel grade. The same steel is written 4140, 42CrMo4, EN 19, 1.7225 and G41400.

Chemical ranges per SAE J404 as printed in ASTM A322 Table 1. Phosphorus and sulphur are maximums.

What is the chemical composition of SAE 4140?

Composition limits in weight percent are carbon 0.38 to 0.43, manganese 0.75 to 1.0, sulphur 0.04 maximum, phosphorus 0.035 maximum, silicon 0.15 to 0.35, chromium 0.8 to 1.1 and molybdenum 0.15 to 0.25.

What is SAE 4140 equivalent to in other standards?

Equivalence means nearest counterpart, not identity: each standards body sets its own composition window, so check the limits of the standard actually named on the order before substituting.

Specifications for SAE 4140

The SAE 4140 designation primarily defines composition. Mechanical properties depend on the applicable end-use specification. ASTM A29 specifies general requirements for hot-wrought carbon and alloy steel bars. ASTM A322 covers alloy steel bars supplied to composition or hardenability requirements. AMS 6349 and AMS 6382 cover 4140 bar and forging stock in the aerospace system. ASTM A193 Grade B7 is the principal specification under which 4140 is consumed for bolting. ASTM A320 Grade L7 is its low-temperature counterpart.

A certificate that identifies only SAE 4140 establishes the steel composition but does not establish its performance. The ordered condition, section size and governing specification determine the required properties.

Mechanical properties

SAE 4140 has no single specified strength. ASTM A193 Grade B7 provides one defined property set for chromium-molybdenum bolting in the quenched and tempered condition. It permits 4140, 4142 and 4145 compositions, including their hardenability-controlled H versions. At diameters up to and including 2.5 inches, the specification requires a minimum tensile strength of 125 ksi, or 860 N/mm squared. It also requires a minimum 0.2 percent yield strength of 105 ksi, or 725 N/mm squared, minimum elongation of 16 percent and minimum reduction of area of 50 percent. Hardness must not exceed 321 Brinell or 35 HRC. The standard reduces these requirements across two additional diameter bands above 2.5 inches. Larger bars cool more slowly at the centre. BS 970 addresses the same effect by associating its conditions with a ruling section.

ASTM A193 Grade B7 specifies a minimum tempering temperature of 1100 degrees Fahrenheit, or 593 degrees Celsius. Tempering at this temperature reduces the attainable strength but increases toughness, relaxation resistance and resistance to embrittlement mechanisms associated with lower tempering temperatures.

Heat treatment and hardenability

The usual heat-treatment route consists of austenitising at about 845 degrees Celsius, soaking until the section reaches a uniform condition, oil quenching and tempering to the required strength. Lower tempering temperatures retain hardness but reduce toughness. Higher tempering temperatures have the opposite effect. The specification or drawing should state either the tempering temperature or the resulting property.

Hardenability describes the depth to which the alloy can harden. The Jominy end-quench test applies a controlled water spray to one end of a standard bar. Hardness measurements at fixed distances from that end produce a curve relating hardness to cooling rate. The 4140H designation and related H grades specify a hardenability band on this curve instead of only a composition range. This provides gear and shaft makers with a defined hardening depth.

SAE 4140 can lose impact toughness when tempered in the low intermediate temperature range. Tempering temperatures are therefore selected below or well above that range. Compliance with a hardness requirement does not establish compliance with a toughness requirement.

Machining and welding

SAE 4140 is supplied in annealed, normalised or pre-hardened conditions. Its machining behaviour depends on the supplied condition. Annealed bar machines freely. Pre-hardened bar in the range commonly used for tool holders and moulds machines predictably but increases tool consumption. A fully hardened and low-tempered part requires grinding. SAE 4140 is not a free-machining steel. Its sulphur content is limited to 0.04 percent because sulphide inclusions that assist chip breaking can also initiate fatigue cracks.

Welding requires preheating. The carbon equivalent calculated from the International Institute of Welding expression at the midpoints of the SAE composition ranges is about 0.78. This exceeds the 0.45 level at which preheat, low-hydrogen consumables and interpass temperature control are required. The procedure uses preheat, maintains the interpass temperature at or above the preheat temperature, uses low-hydrogen consumables and applies a post-weld temper. Welding also changes the quenched and tempered condition within the heat-affected zone.

Applications

Applications include shafts and spindles, axles, gears and gear shafts, connecting rods, moderate-duty crankshafts, tool and die holders, mould bases, drill collars and downhole tools, pump and valve components, forging dies, and high-tensile bolting and studs. Bolting to ASTM A193 Grade B7 is its largest recurring application. B7 studs are the default material for flanged pressure joints in process plants, refineries and pipelines.

Where SAE 4140 is not the right choice

SAE 4140 is unsuitable at the upper end of the strength range. Above roughly 1250 N/mm squared, it becomes notch sensitive and progressively less tolerant of rough fillets and thread roots. At this strength, its toughness is lower than that of a nickel-bearing steel. SAE 4340 provides greater toughness and deeper hardening for such applications.

SAE 4140 is unsuitable when a hard, wear-resistant surface is required over a tough core. It through hardens and does not carburise effectively at 0.4 percent carbon. Gear tooth applications with this requirement use SAE 8620 or EN 36C instead.

SAE 4140 is generally unsuitable for fabrications that are not designed around its welding requirements. Its carbon equivalent of about 0.78 makes each weld dependent on preheat, a defined procedure and post-weld heat treatment. Structures containing many welded joints are usually better specified in a weldable grade.

SAE 4140 has no corrosion resistance. Its chromium content of around one percent cannot form a passive film, which requires roughly ten and a half percent chromium in solution. In wet service, it rusts at the rate of plain steel and relies entirely on paint, plating or oil for protection.

Other alloy steel grades