Nitronic alloys, as their name suggests, consist of a series of nitrogen-bearing austenitic stainless steels. The company Armco Steel before this registered trademark (trade name) becomes the property of AK Steel since 2000 developed the first grade, namely Nitronic 40, in 1961.
The chemical composition of the main alloys of this series is presented in the table below.
Nitronic have quite different properties than those of austenitic stainless steels such as usual grades (e.g. 304, 316L, CF3, CF8M, etc). Thus, the typical properties of these alloys are presented below :
- very high mechanical strength with a ratio strength/density of 3 times that of mild steel;
- superior ductility, up to 40%;
- no sensivity to ductile-brittle transition temperature (DBTT);
- high work hardening, forming ability and impact resistance. Nitronic 60 exhibits a better impact resistance than other grades;
- very good weldability;
- Good resistance to oxidation, to aqueous corrosion and to localized corrosion (pitting and crevice corrosion). Thus, Nitronic 50 offers the best resistance to this type of attack;
- High resistance to stress corrosion. Indeed, Nitronic 33 offers better resistance to this type of attack than other grades. This is due to its low nickel content (~ 3%);
- superior intergranular corrosion resistance than 304 stainless steel grade in chloride mediums as per ASTM G-48 Standard;
- good microstructural stability at high temperatures, up to 1800°F (900°C) ;
- Cavitation and erosion resistance equivalent to cobalt-based alloys but superior to austenitic and duplex stainless steels. Thus, Nitronic 50 and 60 Alloys are considered as substitutes to theses;
- Superior resistance to metal-to-metal wear and to seizing;
- Better behavior to high temperature wear in comparison with hard chromium from 400°F;
- Similar fretting resistance to Inconel 718 at 600°C (1110°F).
The degree of work hardening of Nitronic alloys depends on the rate of deformation rate, the stability of the austenite and the working temperature. Since these alloys have a stable austenitic microstructure, the austenite phase cannot transform certainly into martensite. However, it is reported that the hardening mechanism of the latter consists rather of the formation of a non-magnetic epsilon (Ɛ) phase from austenite. In fact, unlike the austenitic grade 304 which hardens by martensitic transformation (magnetic phase) under the effect of deformation, deformed Nitronic alloys retain their non-magnetic character.
The following graph illustrates the effect of the 60% cold reduction on the consolidation of the mechanical strength properties of different alloys.
In fact, the yield strength increases at a rate of 230, 220, 260, 225 and 335% respectively for the Nitronic grades 33, 40, 50, 60 and stainless steel AISI 304. In addition, it is established that the Nitronic 50 is a foremost choice for use applications requiring a good compromise between mechanical strength, impact and corrosion resistance.
MICROSTRUCTURE AND ROLE OF ALLOYING ELEMENTS
Manganese partially replaces nickel where the addition of 2% Mn (wt.%) manganese is considered as the equivalent effect of 1% Ni(wt.%)for austenite phase stability.
It follows that a partial reduction in the nickel content renders Nitronic alloys less costly than austenitic stainless steel grades of the 3xx series (304, 316, etc.), on one hand, and more competitive than cobalt (Stellite) and nickel alloys on the other hand. In addition to its effect in solid solution strengthening on microstructure, manganese is also a deoxidizing and desulfurizing element, which contributes to prevent hot cracking.
SILICON, Si ;
Silicon is added to certain grades such as Nitronic 60 (4%Si)) for its following effects:
- reinforcement of the chromium oxide layer on the surface and improvement of oxidation resistance;
- solid solution strengthening of the the austentitic phase;
- increase of the metallic friction and adhesion resistance
NITROGEN, N2 :
Nitrogen has the following effects :
- stabilization of austenite;
- incerase of mechanical strength properties (Ys and UTS) at room an very low temepratures (cryogenic use);
- improvement of the stress corrosion racking resistance (SCC);
- increase of localized corrosion resistance (by pitting and crevice). In this respect, chromium and molybdenum act in symbiosis on this trend.
- enhancement of the solubility of in austenite and delay of carbon diffusion towards joint boundaries to prevent intergranular corrosion (IGC). For that, it has been found that with nitrogen contents in the order of 0.30%, Nitronic alloys having up to 0.05%C resist remarkably better to sensitization than austenitic grade AISI 304 that have 0.06%C and residual nitrogen within the range of 0.06-0.08%.
It follows that this effect of nitrogen must be taken into account to mitigate used to prevent the risk of sensitization in Nitronic series, especially in the case of welding of thick joints, which does undergo relatively cooling rates. The rate of cooling rate is not as critical in the welding of Nitronic alloys as in the case of austenitic stainless steels such as grade 304H.
Nitronic Alloys are mainly used in the following applications namely: manufacture of tanks, transport (bumpers, base frame), refrigerated trucks, container ships, armoured vehicles, valve seats, centrifugal pumps, rings, bolts and shafts, fasteners, centrifugal pumps; turbine blades (substitute for CA-6NM), hard surfacing of austenitic stainless steels, etc.
There are several types of wear including abrasion, impact, metal-to-metal friction, erosion, cavitation, corrosive wear, etc. The main factors that influence wear are :
- type of mating surfaces or materials (chemistry, hardness and work hardening);
- geometric factors (surface finish, shape);
- operating parameters (load, temperature, speed, amplitude of movement, motion, travelled distance traveled);
ADHESIVE WEAR – SEIZING
Adhesive wear or seizure is a type of metal-to-metal wear, which results first from the strong interaction and pressure between the chemically compatible mating surfaces. The phenomenon manifests by plastic deformation with formation of intermetallic junctions, called cold micro welds, from the roughness on the contact surfaces subjected to a reciprocal friction movement. This type of wear is very familiar, among other things, to piston and cylinder of engines and it is exacerbated by the following factors:
- smooth contact surfaces with a roughness of less than 10-15 μm. Too smooth surfaces do not retain enough lubricant to reduce friction on the one. On the other hand, very rough surfaces aggravate wear from their protuberances;
- compatibility of materials, either between similar metals or alloys, e.g. SS410 / SS410, or in pair of chemically compatible materials like copper /nickel.
- hardness: an increase of surface hardness by work hardening or surface treatment enhances the resistance to adhesive wear;
- Contact surface and amplitude of the reciprocating motion: oxide debris form a solid tribo-layer and accommodate friction. However, An increase of this amplitude of movement lead to the evacuation of wear debris and aggravates wear;
- Single-phase microstructure;
- contact pressure;
- inadequate lubrication;
REDUCTION OF ADHESIVE WEAR
For metal sliding couples, the adhesive wear can be controlled to some extent by the monitoring of the following parameters:
- lubrication of the contact surfaces;
- reduction of pressure, temperature and also the speed of movement;
- selection of a finish surface in the range of 10 to 70μ inches. Very smooth surfaces do not have a good capacity for retaining wear debris, which serves as a solid lubricant. Likewise, the intermolecular forces exerting on smooth surfaces lead to the formation of junction or cold welds that impede the continuity of the movement and deteriorate the contact surface;
- Increase of the contact area;
- selection of compatible mating surfaces or dissimilar contacts;
- Hardness of the contact surfaces. For the work-hardenable alloys such as Nitronic (eg Nitronic 60), it is the hardness reached on the surface after friction which has to be taken into account. Also, the work hardenability of these alloys makes their seizure resistance superior to that of precipitation hardenable stainless steels (PH) such as the grade 17-4PH.
- Use of hard coatings on the contact surfaces.
- Materials having multiphase or complex microstructures (matrix reinforced by second phases “carbide, nitride, silicide particles, etc”.) offer good resistance against adhesion as compared to single-phase alloys.
The welding and the selection of electrodes and filler metals must be considered according to the type of application (e.g. joining, build-up, repair), the design and the service conditions. Failing to get the matching filler metal, a dissimilar filler alloy shall be used while taking into consideration the following:
- chemical composition;
- mechanical strength of the deposited weld metal and the base metal;
- dilution rate;
- work hardening ability; especially in the case of wear resistant build-up;
- succeptibility to hot cracking;
- corrosion and/or wear resistance
- type of mating surfaces;
All-weld metal from Nitronic alloys retains a stable and tenacious austenitic microstructure. They also contain low nitrogen levels compared to the corresponding base metals. This particularly aims to prevent the risk of porosity. Similar electrodes or filler metals of Nitronic, such as E/ER 209/ 219/240, are produced in such a manner to produce an austenitic deposit with a certain level of ferrite delta (Fe-δ) to counter the likelihood of of hot cracking upon solidification.
Welding of Nitronic alloys does not generally require preheating nor post-weld heating treatment (PWHT), even though the base metal is welded in the work-hardened condition.
The table below shows the standardized products used either as matching or as alternatives electrodes or filler metals.
ELECTRODES / FILLER METALS
WEAR-RESISTANT OVERLAYS (BUILD-UP / HARD SURFACING)
Nitronic alloys are well used as weld deposits in order to reduce friction wear on the contact surfaces of carbon, low alloy steel and austenitic stainless steels. Multilayer deposits can be deposited by different arc welding processes.
It should be noted, however, the specific effects of the choice of materials and their properties on wear of surfaces:
- The resistance of the homogeneous couples is ranked in the following decreasing order: D2, Stellite 6B, Nitronic 60, SS440C (57 RC), Nitronic 32, Nitronic 33, Nitronic 50, Nitronic 40.
*For heterogeneous pair contacts, it has been established that, inter alia, the pair Nitronic 60/SS440C (57 RC) has higher wear resistance than the homogeneous couples of Nitronic 60/Nitronic 60 and AISI 440C/AISI 440C;
- Pairs of similar mating surfaces of Nitronic 60 have a wear resistance that is:
*30% higher than that of homogeneous couple of martensitic stainless grade 440C (57 RC);
*2.5 times higher than that of homogeneous pairs made of Nitronic 32 and Nitronic 50 respectively;
*3 times greater than that of a homogeneous pairs of Nitronic 50
It appears that Nitronic 60 is a good candidate for building-up wearing surfaces deposits on wear surfaces to reduce friction and adhesion. Likewise, Nitronic alloys are revolutionizing different Stellite and nickel-based alloys in industrial applications involving friction wear.