1. Introduction to High Nitrogen Austenitic Stainless Steel

High Nitrogen Austenitic Stainless Steel (HNASS) represents a new generation of resource-saving, high-performance green advanced materials. This innovative steel utilizes cost-effective nitrogen to replace expensive and scarce nickel, resulting in exceptional mechanical properties and corrosion resistance.

The excellent performance of HNASS is closely linked to its nitrogen content. Nitrogen acts through multiple strengthening mechanisms including solid solution strengthening, grain refinement, strain hardening, and precipitation hardening. As an austenite stabilizing element, nitrogen’s ability to stabilize austenite is 18 times that of nickel, making it an economically attractive alternative.

However, significant technical challenges remain in the manufacturing process, including imprecise control of nitrogen enrichment levels, nitrogen gas precipitation during solidification causing porosity, and coarse nitride precipitation during hot working—all of which limit large-scale development and application.

2. Global Development History and Current Status

2.1 International Development

The history of nitrogen-containing stainless steel dates back to the early 20th century. In 1912, the beneficial effects of nitrogen on steel were first discovered when researchers explored nitrogen’s influence on stainless steel mechanical properties, revealing its exceptional ability to stabilize austenite.

World War II led to nickel shortages and price surges, accelerating research into nitrogen as a nickel substitute. This period saw the development of the AISI 200 series stainless steel. In 1977, researchers discovered that chromium introduction could increase nitrogen solubility in the matrix, and raising furnace nitrogen pressure could significantly enhance nitrogen dissolution.

Recent advances in additive manufacturing have opened new possibilities. Researchers have successfully used laser powder bed fusion technology to produce nickel-free HNASS with nitrogen content (w[N]) reaching 0.87% under atmospheric pressure, demonstrating excellent comprehensive properties.

2.2 Domestic Progress

Research in China began later compared to international efforts, starting in the 1950s when researchers first studied nitrogen’s effects on stainless steel structure and properties. The 1970s saw rapid development in petrochemical and aerospace sectors, driving requirements for high-performance HNASS. During this period, a nitrogen-containing duplex stainless steel 0Cr17Mn14Mo2N was successfully developed, demonstrating excellent corrosion resistance in various media.

In 2006, the 8th International Conference on High Nitrogen Steels was held in China, facilitating extensive international exchange. Recent years have witnessed significant achievements, including the development of 18Cr18Mn2Mo0.9N HNASS with nitrogen content up to 1.21%, exhibiting excellent room temperature mechanical properties, ductility, and pitting corrosion resistance.

3. Manufacturing Methods: Melting-Casting Processes

The dissolution behavior of nitrogen in stainless steel is the primary limiting factor for traditional melting-casting methods. Under atmospheric pressure, nitrogen solubility in molten steel is relatively low, insufficient to meet HNASS requirements. Additionally, as the steel solidifies through the δ-Fe phase region (which has even lower nitrogen solubility), nitrogen tends to escape, causing porosity defects.

3.1 Larger Pool Method (BSB)

The Larger Pool Method encompasses various production approaches including counter-pressure casting, gravity casting, and integral casting. This method enhances nitrogen content primarily by strengthening gas-liquid reactions through increased nitrogen pressure in the melt pool and enhanced convection intensity.

Under stirring action, intense convective reactions occur within the melt pool, shortening nitrogen diffusion distances and accelerating equilibrium times. The counter-pressure casting technique successfully addresses nitrogen escape during solidification by maintaining high pressure throughout the casting process, effectively closing the δ-Fe ferrite phase region.

3.2 Pressurized Induction Melting (PIM)

Pressurized Induction Melting utilizes electromagnetic induction to heat and melt raw materials while the alternating magnetic field stirs the melt pool, creating convection effects that accelerate nitrogen diffusion. Combined with pressurization, this shortens saturation time for rapid nitrogen enrichment.

Researchers have achieved nitrogen content of 0.689% using this method and discovered that at higher nitrogen solubility levels, equilibrium solubility deviates from Sievert’s law. Compared to conventional 316 stainless steel, PIM-produced HNASS demonstrates superior room temperature mechanical properties, finer grain structure, and more stable austenite phase.

3.3 Pressurized Ladle Nitrogen Blowing

This technology involves pressurized bottom-blowing of nitrogen through the ladle for HNASS production. The bottom-blown nitrogen stirs the melt pool, homogenizing temperature and composition while increased nitrogen pressure enhances saturation solubility. Qualified high-nitrogen steel can then be processed using traditional casting methods, reducing production costs.

Using AOD furnace bottom-blown nitrogen alloying, researchers have achieved nitrogen content of 0.547%, and after LF refining, produced HNASS with 0.68% nitrogen.

3.4 Pressurized Electroslag Remelting (PESR)

PESR uses heat generated from current passing through high-resistance slag to melt electrode materials. Molten steel droplets pass through high-temperature slag into the metal pool and are rapidly cooled by the crystallizer into ingots. This process maintains high pressure throughout while continuously adding nitrogen-containing alloys.

PESR can produce HNASS with relatively high nitrogen content and effectively reduces nitrogen escape through rapid solidification mechanisms and high nitrogen partial pressure during solidification. Research shows that PESR-produced HNASS demonstrates superior fatigue resistance compared to other production methods.

3.5 Pressurized Plasma Arc Melting (PARP)

PARP utilizes plasma arc as the heat source for melting, remelting, and refining metals. The plasma arc ionizes nitrogen gas into nitrogen atoms, accelerating surface saturation. While this method achieves high nitrogen content without requiring nitrogen-containing alloy additions, its high power consumption and complex equipment limit development.

4. Manufacturing Methods: Powder Metallurgy

Powder metallurgy offers distinct advantages for HNASS production. In solid-state austenite, nitrogen solubility is higher than in molten steel, enabling high nitrogen content at relatively low nitrogen pressures and temperatures. This method produces homogeneous compositions with convenient nitrogen content control and achieves near-net-shape forming with material savings.

4.1 Mechanical Alloying (MA)

Mechanical alloying involves weighing and mixing powders according to pre-designed compositions, then ball milling in high-energy mills. During milling, metal powders undergo repeated cold welding and fracturing from intense collisions with grinding balls, increasing surface area for nitrogen adsorption and accelerating nitriding.

Researchers have successfully produced Fe-Cr-Mn-N stainless steel powder with nitrogen content reaching 3.51% after 30 hours of ball milling, achieving single austenite phase with 92% relative density after sintering and quenching. The MA process achieves high nitrogen content because nitrogen solubility in solid austenite far exceeds that in liquid steel, avoiding the δ-Fe phase transition and nitrogen escape issues.

4.2 High-Pressure Nitrogen Gas Atomization

This method produces HNASS powder by melting high-nitrogen steel and directly atomizing it under high-pressure nitrogen jet impact. The molten steel shatters into droplets of varying sizes that spheroidize under surface tension and rapidly cool into powder.

Powders produced by this process exhibit high sphericity, low oxygen content, and no composition segregation, with demonstrated large-scale production capability. Research has achieved nitrogen content of 0.402% without adding nitrogen alloys, and average powder nitrogen content of 0.736% using atmospheric melting combined with nitrogen atomization.

4.3 Plasma Rotating Electrode Process (PREP)

PREP uses high-temperature plasma from plasma guns to melt the end face of rapidly rotating consumable electrode rods. The molten layer at the rod end is thrown off by centrifugal force, and the droplets cool and spheroidize under surface tension to form metal powder.

PREP-produced powders exhibit high sphericity, no hollow particles, and meet quality requirements for hot isostatic pressing, spark plasma sintering, and additive manufacturing. The plasma gun ionizes nitrogen into ions and atoms while melting the electrode surface, producing powder with high nitrogen content, excellent purity, and good flowability.

4.4 Solid-State Powder Nitriding

Solid-state nitriding achieves higher nitrogen content by exploiting the fact that nitrogen solubility in solid austenite is much greater than in liquid steel. Methods include sintering nitriding (placing powder in nitrogen atmosphere), rotary furnace nitriding (using active nitrogen from ammonia decomposition), and fluidized bed nitriding (suspending particles for increased contact area).

Using tube furnace solid-state nitriding under nitrogen-hydrogen atmosphere, researchers have produced FeCrMnN HNASS powder with nitrogen content reaching 2.62% and yield strength up to 1,111 MPa after forming. Fluidized bed nitriding has increased powder nitrogen content from 0.02% to 0.66%.

5. Powder Forming Technologies

Two primary approaches exist for powder metallurgy HNASS production: (1) preparing high-nitrogen powder first through mechanical alloying or atomization, then forming through hot pressing, HIP, or plasma sintering; (2) pressing powder into desired shapes first, then achieving target nitrogen content through sintering-nitriding parameter control.

5.1 Hot Isostatic Pressing (HIP)

HIP densifies powder through simultaneous heating and pressurization. Powder is placed in evacuated cans, then nitrogen or other pressure media is introduced into the HIP equipment. Under high temperature, expanding gas applies equal force from all directions, sintering and densifying the powder.

Research has achieved 96% density with ultimate tensile strength of 935 MPa at 1,150°C forming temperature, demonstrating excellent corrosion resistance compared to 316 stainless steel. However, HIP requires pre-encapsulation of powder materials and faces challenges including nitride coarsening during sintering and size/shape limitations.

5.2 Spark Plasma Sintering (SPS)

SPS densifies powder through combined heating and pressure by passing current through powder in graphite crucibles, causing self-heating. With fast heating rates and short forming times, SPS is an important densification process for powder metallurgy HNASS.

SPS achieves higher density at lower temperatures—researchers have produced fully austenitic HNASS with similar densification at 300-500 K lower sintering temperatures compared to conventional sintering. Studies confirm that grain boundary strengthening and solid solution strengthening are the primary mechanisms for excellent mechanical properties in SPS-produced HNASS.

5.3 Metal Injection Molding (MIM)

MIM is a near-net-shape technology including feedstock preparation, injection molding, debinding, and sintering. This technology is widely applied in electronics, medical/chemical industries, aerospace, and defense sectors.

During debinding, binder removal creates void channels that increase nitrogen-powder contact area, shorten diffusion distances, and enable more uniform nitrogen distribution. Research has achieved 99% relative density with nitrogen content of 0.78% using optimized sintering processes, with yield strength reaching 580 MPa after solution treatment. At 1,200°C sintering for 2 hours, optimal yield and tensile strengths of 1,087 and 1,113 MPa respectively have been obtained.

5.4 Cold Pressing and Sintering

Cold pressing followed by sintering involves mixing powder with binder, pressing to form shapes, then high-temperature nitriding. Researchers have achieved nitrogen content up to 4.02% by optimizing nitrogen pressure, sintering temperature, and holding time. This process can continue nitriding during sintering, with binder voids expanding nitrogen contact area for enhanced penetration.

5.5 Additive Manufacturing (AM)

Additive manufacturing creates parts “bottom-up” and “layer-by-layer” using 3D computer models. AM can produce complex shapes impossible with traditional processes while addressing material waste issues. Key technologies include Selective Laser Sintering (SLS), Electron Beam Melting (EBM), Electrochemical Deposition Manufacturing (EDM), and Selective Laser Melting (SLM).

Compared to traditional production processes, additive manufacturing technology will accelerate HNASS development and is expected to become a primary method for efficient high-performance product preparation.

6. Strengthening Mechanisms of Nitrogen

Nitrogen in HNASS improves material properties primarily through solid solution strengthening, grain refinement, strain hardening, and precipitation strengthening mechanisms.

6.1 Solid Solution Strengthening

Nitrogen atoms occupy octahedral interstitial positions in face-centered cubic lattices, causing lattice distortion that substantially increases material strength. Solution treatment involves heating samples to specific temperatures, holding, and water quenching, which re-dissolves precipitated nitride particles into austenite to improve mechanical properties.

Research shows that each 0.01% increase in nitrogen content improves 316LN tensile and yield strength by approximately 9 MPa and 7 MPa respectively. HNASS with higher nitrogen content achieves higher yield strength after solid solution strengthening, demonstrating clear nitrogen solid solution effects.

6.2 Grain Refinement Strengthening

Grain refinement increases strength by reducing grain size. During recrystallization, nitride second-phase particles inhibit austenite grain growth, effectively reducing grain size and increasing grain boundary area, thereby enhancing strength and improving toughness.

The Hall-Petch coefficient K increases with nitrogen content. Research shows that when w[N] increases from 0.008% to 0.34%, K value rises from 363 to 1,895 MPa·μm^1/2, with grain refinement contributing 163 MPa to yield strength. Grain size can decrease below 10 μm with increasing nitrogen content.

6.3 Strain Hardening

Nitrogen’s strain hardening effect occurs primarily during cold deformation, with deformation mechanisms changing as strain increases. As stress increases, deformation mechanisms transition from planar slip at low stress to twinning at high stress—the primary reason for enhanced strain hardening in HNASS.

During low-strain deformation, dislocation density continuously increases and accumulates at grain boundaries, creating greater mutual obstruction. Nitrogen’s presence significantly reduces stacking fault energy and forms short-range order, causing strength to continuously increase with deformation. Studies show that at 77 K, experimental steels exhibit dimple fracture with excellent ductility.

6.4 Precipitation Strengthening

Nitrogen can form stable CrN and Cr₂N nitrides in the HNASS matrix. When these nitrides and carbides exist as fine second-phase particles dispersed in the matrix, they obstruct dislocation movement and inhibit grain growth, significantly improving strength.

Research on aging treatment effects shows that early aging stages produce numerous discrete spherical nitrides that enhance both strength and hardness. After 70% cold rolling and annealing at 800°C, microstructures contain abundant abnormally dispersed fine spherical nitride precipitates, with yield strength increasing by 228 MPa. As aging temperature and time increase, microhardness initially increases then decreases.

7. Current Challenges and Future Outlook

Despite significant progress, HNASS research and development still faces considerable gaps compared to leading countries. Based on current theoretical foundations, several key challenges and future directions can be identified:

8. Frequently Asked Questions