Development of AISI 416 Stainless Steel in a Small Steel Mill (Part One)

In many applications, the delivery conditions of materials make very strict demands on mechanical properties such as annealed hardness, strength under quenching and tempering conditions, as well as corrosion resistance and oxidation resistance. This article outlines the melting process of martensitic stainless steel AISI416, which is the steel with the highest machining performance level among all stainless steel varieties. By smelting in an induction furnace and adjusting the operational process parameters in the forging process, AISI416 stainless steel has been successfully developed. AISI416 stainless steel is suitable for heat treatment and easy to process, thereby reducing its machining cost. The low friction performance of this steel reduces wear and jamming, which means it is considered the most economical stainless steel variety in many applications, and it may even replace AISI410 in many future applications.

AISI416 belongs to the ferrite-martensite steel, which is very similar to AISI410. Its chromium content is about 0.5% more than AISI 410 and contains at least 0.15% sulfur. Carbon provides the required strength, chromium improves its corrosion resistance, and sulfur improves its machining performance. Due to the many influencing factors, such as microstructure, grain size, heat treatment, chemical composition, hardness, tensile and yield strength, and other physical properties including elastic modulus, thermal conductivity, thermal expansion coefficient, and work hardening characteristics, it is difficult to predict its machining performance. Other important influencing factors also include operating conditions, tool material and geometry, and machining process parameters.

Sulfur is completely dissolved in the steel in the form of FeS, with a melting point of 1190°C, but at low-temperature equilibrium, it forms an FeFeS eutectic with a melting point of 983°C. Therefore, when heating for forging, this low-temperature eutectic will precipitate and melt at the grain boundaries, causing the steel to become hot-brittle. We recommend converting FeS to MnS, as MnS is completely soluble in Fe. MnS melts around 1400°C, but unlike FeS, it does not form any low-melting-point eutectics at grain boundaries when the oxygen content in the molten pool is high.

Killed steel refers to steel that is fully deoxidized by adding additives before casting, so that no gas precipitation occurs during the solidification process. It is characterized by high chemical homogeneity and no porosity defects. This kind of steel is called "killed steel" because it solidifies quietly in the mold without bubbling. For easy identification, "K" is used as a marker. In this study, aluminum is used as a deoxidizer; aluminum reacts with the dissolved gas in the steel to form aluminum oxide. The precipitation of aluminum oxide provides additional benefits for pinning grain boundaries, which prevents grain growth during heat treatment.

AISI416 is a ferrite-martensite type of steel, with both delta-ferrite and austenite present during the forging process. Delta-ferrite is harmful to hot working performance, especially at low temperatures, so the content of ferrite-forming elements such as Si and Cr should be as low as possible. However, to maintain the yield of Cr, the Si content should be kept above 0.30%. Sulfur can be added to the steel using sulfur melting bars, FeS (pyrite), or MnS (pyrrhotite). Direct use of sulfur is not recommended due to the risk of air pollution and fire. The addition of sulfur is completed in the steel pot, with Al (Al concentration in the molten pool is 0.030%) for pre-deoxidation.

Due to the small size of the steel-casting pots in most small steel mills, we decided to add only half of the amount in the furnace after the completion of Al reduction and before the steel is poured. If the additive is in powder form, it can be placed in a thin-walled tin can for easy addition and absorption. Both preparations have a sulfur content of 35% to 45%. They do not contain any detectable harmful impurities. The recovery rate of S is expected to reach 50% to 60%, and due to the absence of desulfurization alkaline slag, the recovery rate of sulfur may be closer to the upper limit. The initial target sulfur content is 0.35% (not 0.4%). The target sulfur content will be adjusted to a lower value in the subsequent casting process. Adding 1 kg of FeS and MnS per ton of steel will lower the steel temperature by 0.2°C. To achieve a sulfur content of 0.4%, the temperature is estimated to drop by 0.8°C. The detailed recommended process steps are as follows.

(1) Control the chromium at the lower limit of the specified range (12% to 14%); (2) Keep low silicon, but not less than 0.30%. (3) Keep sulfur at 0.15% to 0.2%; (4) Mn is 1.10% to 1.21%; (5) Al ≥ 0.03%; (6) It is recommended to add half the amount of FeS or MnS to the furnace after Al reduction and before steel pouring, with the rest added to the steel pot. If the additive is in powder form, place it in a thin-walled tin can, but the actual operation should be added during the steel pouring process; (7) After the steel ingot is demoulded, air-cool it and quickly put it into the furnace for annealing.

About the minimum content of manganese, there is no unified opinion. The standard setters also allow the maximum content of Mn to be increased to 1.25% within a certain range. Steel mills can keep the Mn/S ratio at 5:1, that is, keep the S content at 0.20% to 0.25%, and the Mn content at 1.20% to 1.25%. From a thermodynamic point of view, it is impossible to reduce the content of FeS to zero, but it can be controlled within a lower range. When the steel is slowly heated or cooled in the temperature range of 950 to 1000°C, this grain boundary FeS will liquefy (its melting point is 983°C). If the steel is kept or slowly heated/cooled in this temperature range, the material will tend to reach an equilibrium state, and at this time, a FeS melt will form, which will destroy the continuity of the metal. Therefore, it should quickly pass through this temperature range. After the steel ingot is demoulded, it should be air-cooled and then quickly put into the furnace for annealing. The annealing temperature should be below 900