AISI 416

What is AISI 416?

AISI 416 is a free-machining martensitic stainless steel containing 12.0 to 14.0 percent chromium, up to 0.15 percent carbon and at least 0.15 percent sulphur. Atlas Steels rates its machinability at about 85 percent of that of a free-machining carbon steel. This is the highest rating among stainless steels and exceeds that of free-machining austenitic AISI 303.

AISI 416 can be hardened by austenitising, quenching and tempering. Its response to heat treatment is similar to that of AISI 410. This permits machining on automatic equipment before heat treatment to the required working hardness. Austenitic grades do not provide this combination of machinability and hardenability.

The added sulphur distinguishes AISI 416 from AISI 410. Manganese sulphide inclusions assist chip breaking but reduce corrosion resistance, weldability and formability below those of AISI 410. High-sulphur free-machining grades such as AISI 416 are unsuitable for marine and other chloride exposure.

Laxcon Steels lists AISI 416 in its grades reference as a martensitic grade. The same steel is written 416, SUS 416, 1.4005 and S41600.

What is the chemical composition of AISI 416?

Composition limits in weight percent are carbon 0.15 maximum, manganese 1.25 maximum, sulphur 0.15 minimum, phosphorus 0.06 maximum, silicon 1.0 maximum, chromium 12.0 to 14.0 and zirconium plus molybdenum 0.6 maximum.

What is AISI 416 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.

What is the PREN of AISI 416?

Computed by Laxcon Steels from the composition above as PREN = Cr + 3.3Mo + 16N, the relationship published in Practical Guidelines for the Fabrication of Duplex Stainless Steels (International Molybdenum Association, third edition, 2014). The published relationship is given for austenitic and duplex stainless steels, so this figure extends it: read it against other martensitic grades rather than across families. The first figure uses the minimum specified chromium, molybdenum and nitrogen, so it is the floor for a conforming heat rather than a typical value; the second uses the maxima, so the two together are the width of the band one compliant grade allows. PREN ranks resistance to chloride pitting and nothing else: it does not predict crevice corrosion, stress corrosion cracking or service life, and it is not a substitute for a corrosion test. The pitting resistance reference carries the full table and the caveats.

ASTM A582 requirements for AISI 416

ASTM A582 governs free-machining stainless steel bar. ASTM A276 does not govern this product category. ASTM A582 specifies hardness rather than strength for AISI 416. Annealed Condition A has a maximum hardness of 262 HB. Typical values published by Atlas Steels are about 517 MPa tensile strength, 276 MPa 0.2 percent proof stress and 30 percent elongation in 50 mm. These values are not guaranteed minima and vary with composition and heat treatment. Hardened and tempered Condition T has a specified hardness range of 248 to 302 HB. This is also a hardness requirement rather than a strength requirement.

A guaranteed minimum yield strength must therefore be stated as a supplementary requirement agreed with the mill. A grade-only order specifies a hardness range and a machining rate.

Heat treatment and mechanical properties

Full annealing requires holding at 815 to 900 degrees Celsius for half an hour per 25 mm of thickness. Cooling proceeds at no more than 30 degrees Celsius per hour to 600 degrees Celsius, followed by air cooling. Sub-critical annealing uses 650 to 760 degrees Celsius followed by air cooling and produces the best machinability. Hardening requires heating to 925 to 1,010 degrees Celsius, oil quenching and tempering for the required mechanical properties.

The Atlas Steels tempering curve shows the resulting property range. At a tempering temperature of 300 degrees Celsius, AISI 416 has about 1,350 MPa tensile strength, 1,050 MPa 0.2 percent proof stress, 10 percent elongation, 410 HB hardness and 50 J Izod impact energy. At 600 degrees Celsius, the corresponding values are about 870 MPa, 720 MPa, 20 percent, 280 HB and 45 J. At 700 degrees Celsius, they are 710 MPa, 500 MPa, 22 percent, 210 HB and 65 J.

Tempering between 400 and 580 degrees Celsius must be avoided because ductility is poor in this range. At 500 degrees Celsius, the same table gives 1,400 MPa tensile strength and 420 HB hardness, but only about 15 J Izod impact energy. Hardness alone therefore does not establish adequate toughness for material tempered in this range.

Machining characteristics

Sulphur combines with manganese to form manganese sulphide inclusions within the steel. Under a cutting edge, these inclusions act as internal stress raisers and divide the chip into short pieces. The sulphide also spreads across the tool face as a solid lubricant. These effects reduce cutting force and built-up edge, improve surface finish and extend tool life. Machinability is highest in the sub-critical annealed condition.

The inclusions elongate in the rolling direction. Transverse ductility and impact toughness are therefore lower than the corresponding longitudinal properties. A component loaded across the bar axis is weaker than the same component loaded along the axis.

Corrosion, temperature resistance and welding

The pitting resistance equivalent number is 12.0 at the specified composition minima and 14.0 at the maxima. Practical resistance is lower than these values suggest because each sulphide inclusion can initiate a pit. AISI 416 has useful resistance to dry atmospheres, fresh water, mild alkalis and mild acids. Its resistance is lower than that of 17 percent chromium ferritic AISI 430. Maximum corrosion resistance occurs in the hardened condition with a smooth surface finish.

Scaling resistance is fair in intermittent service to about 760 degrees Celsius and in continuous service to about 675 degrees Celsius. The tempering temperature of the component generally sets a lower practical service limit. Weldability is poor. When welding is unavoidable, 410 low-hydrogen electrodes are used with a preheat of 200 to 300 degrees Celsius. Annealing, re-hardening or stress relief at 650 to 675 degrees Celsius follows immediately. Austenitic 309 filler is preferable when the joint does not require hardness.

Applications and designations

Applications include valve parts, pump and motor shafts, automatic screw machined components, washing machine parts, bolts, nuts, studs and gears. AISI 416 is also used in solenoids and magnetic-circuit components. Martensitic and ferritic stainless steels are ferromagnetic in every condition, while austenitic grades are not ferromagnetic in the annealed condition. The nearest European counterpart is EN material number 1.4005, with the name X12CrS13. The UNS designation is S41600. The Japanese designation is SUS 416, also written by purchasers as SUS416 or SS416.

Where AISI 416 is not the right choice

AISI 416 is unsuitable for chloride-bearing environments. Marine and coastal exposure, brine, chlorinated water and salt spray cause rapid attack that begins at the sulphide inclusions. Polishing and passivation do not remove this vulnerability. AISI 410 retains hardenability without the same free-machining sulphur addition. AISI 431 provides greater corrosion resistance while retaining hardenability. Austenitic grades apply where hardening is unnecessary.

AISI 416 is unsuitable for welded fabrication. Sulphur promotes solidification cracking in the weld pool, while martensitic transformation can crack the heat-affected zone. Preheating and post-weld treatment do not eliminate these limitations. The grade is also unsuitable for cold-forming operations such as sharp bending, heading and deep drawing because of its reduced ductility.

AISI 416 is unsuitable for sub-zero service because martensitic grades undergo a ductile-to-brittle transition. It is also unsuitable for service above the component's tempering temperature because continued tempering reduces hardness. Tempering between 400 and 580 degrees Celsius is prohibited because of reduced toughness. Where high machinability is required without hardening, AISI 303 provides greater corrosion resistance and better availability within the austenitic family.

Other martensitic grades