GENERAL CHARACTERISTICS OF 4140 ALLOY STEEL
AISI/SAE 4140 grade is a versatile alloy with good atmospheric corrosion resistance and reasonable strength. It shows good overall combinations of strength, toughness, wear resistance and fatigue strength.
APPLICATIONS
This alloy finds many applications as forgings for the aerospace and oil and gas industries, along with myriad uses in the automotive, agricultural and defense industries, Typical uses are forged gears and shafts, spindles, fixtures, jigs and collars.
FORGING
Forging of this steel should be carried out between 2200 and 1650 º F (1200 and 900 º C). The lower the finishing temperature from forging, the finer will be the grain size. This alloy should ideally not be forged below 1650 º F (900 º C) and should be slow cooled after forging.
HEAT TREATMENT
Heat treatment is carried out after hot working to render the steel suitable for machining, and to meet the mechanical property ranges specified for the steel’s particular applications.
ANNEALING
Forgings of 4140 grade may be annealed by transferring the parts straight from the forging operation to a furnace held at a suitable temperature, between 1450 and 1550 º F (790 and 840 º C), holding for a suitable time then furnace cooling, forming a structure suitable for machining. This kind of treatment is best used for parts with simple shapes. If some areas of a forging will finish much colder than others then a uniform structure will not be obtained, in which case a spheroidizing anneal at around 1380 º F (750 º C) may be used. It is safe to say that experience alone will decide the best type of annealing treatment to be used prior to machining.
NORMALIZING
This process is defined as heating a steel to a temperature above the ferrite to austenite transformation range and then cooling in air to a temperature well below this transformation range. This treatment may be carried out on forged products as a conditioning treatment prior to final heat treatment. Normalizing also serves to refine the structure of forgings that might have cooled non-uniformly from their forging operation. The nominal normalizing temperature for 4140 grade is 1600 º F (870 º C), but production experience may necessitate a temperature either 50 º F (10 º C) above or below this figure. In fact when forgings are normalized before, say, carburizing or hardening and tempering, the upper range of normalizing temperatures is used. When normalizing is the final heat treatment, the lower temperature range is used.
HARDENING
This heat treatment results in the formation of martensite after quenching, hence a great increase in hardness and tensile strength together with some loss of ductility. The steel should be austenitized at 1500 to 1550 º F (815 to 845 º C), the actual temperature being a function of chemical composition within the allowed range, section size and cooling method. Austenitizing should ensure all micro-constituents in the steel are transformed to austenite. Smaller sections of 4140 might be quenched in oil, heavier sections in water.
Tempering is carried out to relieve stresses from the hardening process, but primarily to obtain the required mechanical properties. The actual tempering temperature will be chosen to meet the required properties, and in many cases will be a matter of trial and error.
MACHINABILITY
The alloy is readily machinable. Simple shapes might be machined following a normalizing treatment, whereas more complex shapes will require annealing. . At the carbon level of this grade, a structure of coarse lamellar pearlite to coarse spheroidite is normally optimum for machinability
WELDABILITY
This grade has good weldability and may be welded using any commercial method. Parts should be preheated before welding at around 1100 º F (590 º C) and stress relieved after. Parts in the hardened and tempered condition should not be welded since mechanical properties will be adversely affected. Parts should be welded in the annealed condition only.
COLD FORMABILITY
Any cold forming on this alloy should be carried out on material with a spheroidized structure. Further heat treatment, hardening and tempering, is carried out where applicable on the finally formed part