1.4109 forgings X70CrMo15 high carbon martensitic stainless steel, forged to drawing in Jiangyin, China

1.4109 and X70CrMo15 are the number and the name of the same steel in EN 10088-3, the European delivery standard for stainless bars, rods, sections and semi-finished products. The grade also appears in EN 10088-1 and is one of the steels listed in ISO 7153-1 for surgical and dental instruments. Older drawings from German sources carry the name X65CrMo14, which refers to the same material.

No standard outside Europe combines the same chromium band with a required molybdenum addition, so the designations below are near equivalents. Use them to find material. Check the certificate against the standard your drawing names before you accept any of them.

Carbon at 0.60 to 0.75 % is far above the level at which chromium carbides form. A large fraction of the chromium therefore sits in carbide instead of in solution. Those carbides give the abrasion resistance and the edge holding. They also hold the corrosion resistance down to a moderate level, and they make melt quality a larger factor here than it is in a low carbon stainless.

Most stockholders treat these two as one steel. On general wear parts the substitution usually passes without trouble. On a controlled drawing it does not. The carbon bands are identical, the chromium bands touch at a single point, and molybdenum is a requirement in one standard and a permitted residual in the other.

EN 10088-3 specifies this grade in the annealed condition only, as a hardness ceiling and a tensile ceiling. There is no specified proof strength, elongation or impact value, because the standard treats 1.4109 as a steel the buyer hardens. Service properties are produced by the hardening cycle, and the delivery condition says nothing about them.

The hardened figure for this grade is set by ISO 7153-1, which lists X70CrMo15 among the martensitic steels for surgical instruments at 55 to 60 HRC, equivalent to 610 to 720 HV.

No published tempering curve exists for 1.4109 on its own. The table below is manufacturer data for AISI 440A, which carries the same carbon band and is the closest grade with a full curve. Take the shape of the curve from it and treat the absolute figures as indicative. 1.4109 runs one to two points of hardness above 440A on the same cycle. It holds about 2 % less chromium, so less carbon is locked into carbide and more of it stays in solution to form martensite.

Four temperature windows govern this grade, and one band between them has to be avoided. The chart puts them on a single scale. The table below it repeats the same numbers with holding and cooling practice.

Choosing the hardening temperature

Austenitising at the low end of the window leaves more chromium carbide undissolved. That keeps the grain fine and the toughness reasonable, but it leaves less chromium in solution, so both hardness and corrosion resistance drop slightly. Austenitising at the high end dissolves more carbide and raises both, until retained austenite starts to cost hardness instead. Instrument and bearing work normally takes the upper half of the range with a sub-zero step after the quench. Heavy wear parts are safer in the lower half with a longer soak.

1.4109 is a high carbon steel carrying about 15 % chromium. At the top of the temperature range it goes hot short. At the bottom it cracks. The window between those limits is narrow, and the cooling that follows the last blow counts for as much as the forging.

In a high carbon martensitic stainless steel the primary carbides form during solidification, in a coarse network. Nothing in later heat treatment removes that network. Only hot deformation breaks it up, and the degree of break-up follows the forging reduction. A ring rolled from a forged and punched preform has its carbides broken and its grain flow following the circumference. A ring parted off a rolled bar and bored out has neither.

The difference turns up in service as a chipping edge on a blade, early spalling on a bearing race, or a pit starting at a carbide interface on a valve seat. Our certificates carry a forging ratio and a grain flow direction for that reason.

The pitting resistance equivalent sits near 17 at mid range chemistry, which puts 1.4109 well below the 18 % chromium austenitic grades and far below duplex. What it offers is corrosion resistance combined with a hardness those grades cannot reach.

Full corrosion resistance exists only after hardening and tempering, when the maximum amount of chromium is in solution. Annealed material has its chromium locked into carbide and corrodes noticeably faster, so parts should not be left in service in the annealed state. Surface condition counts as well. A ground or polished surface resists attack far better than an as-machined one, because pitting starts at surface defects and at exposed carbides.

Machining

Machine in the annealed condition. At 250 to 280 HB the grade cuts acceptably with carbide tooling at moderate speed and a positive feed, and it produces long stringy chips that need a chip breaker. After hardening, finishing is done by grinding. Leave a machining allowance for the distortion and the size change that the hardening cycle produces, and specify any close tolerance features to be finished after heat treatment.

Welding

Do not plan to weld this grade. The heat affected zone hardens as it cools in air and cracks, and a post-weld treatment long enough to soften it also runs the part through the embrittlement range. Where a weld is unavoidable, preheat to 260 °C, hold at 732 to 760 °C for six hours afterwards, and cool in the furnace. Forging the component from solid removes the question, and that is how we make these parts.

Every item below is forged from one piece of steel melted at this works. No castings and no welded assemblies.

Standard delivery is annealed, 280 HB maximum, as forged with machining allowance or rough machined. Proof machined surfaces are supplied where ultrasonic examination is required, since the inspection is carried out in the annealed condition. Hardened and tempered delivery to an agreed hardness, and finish machining to drawing, are both available.

The steel is melted on site, so the analysis is verified at our own furnace and not copied from a supplier certificate. The melt number follows the part through forging, heat treatment, machining and shipment.