Special Quality Welding Filler Metals for High Strength, Precipitation-Hardening, Stainless Engineering Steel Alloys
Advanced engineering construction industries have an increasing requirement for exceptional very high strength, corrosion-resistant, economical alloys that are formable and readily available. A range of noteworthy precipitation-hardening stainless engineering alloys, as a specialized metallurgical class, is able to develop a combination of superior tensile strength with corrosion and oxidation resistance.
These premium alloys offer the designer a unique combination of fabricability and challenging metallurgical characteristics not found in any other series of material. This combination of sought-after mechanical and chemical material attributes encourages the precipitation-hardening stainless steel alloys to be applied to military, nuclear, aerospace, transport and other advanced and critical applications for instance.
While designed originally for wrought applications in round sections, forgings and shapes, precipitation-hardening stainless steels now also find growing use in the flat rolled form. The precipitation-hardening stainless steel series is based upon iron- nickel- chromium and incorporates one or more extra elements such as copper, aluminium, molybdenum, niobium, carbon and titanium.
During a relatively straightforward ageing treatment of the fabricated components, separation from the matrix, of a second ultra-fine coherent copper-rich phase or a coherent nickel-aluminium phase provides the potent strengthening mechanism for some of this alloy series.
U.S. Welding Corp. has been active in producing special MC-GRADE®, high purity welding filler metals for the range of precipitation- hardening stainless steel grades since their widespread inception in the mid 1970s for military use. In all the range of the precipitation hardening class of stainless engineering steels, USWC has successfully pioneered the appropriate matching and adjusted-chemistry filler metals for all types of applications to the benefit of the entire fabrication industry. USWC produces high quality filler metal, MC-GRADE® versions, of the martensitic precipitation-hardening grades of 17-4PH, FV520, PH13-8Mo and 15-5PH.
The aerospace quality grades of these filler metals, enhances ductility and toughness properties in high strength welded components that are aged to peak property levels. Large volumes of MC-GRADE® 17-4PH, AMS 5803 and AMS 5825 have been manufactured by USWC over a period of 35 years. MC-GRADE® filler wires are produced by roller die compression and are lubricant-free. There is control over subcutaneous processing contamination, which can be a feature of commercial wire drawn welding filler alloys. MC-GRADE® high purity filler metals are ideally suited for Wire + Arc Additive Manufacturing ( WAAM ) welding applications.
Classes of Precipitation-Hardening Stainless Steels
There are three classes of Precipitation-Hardening Stainless Steels based upon the ambient temperature matrix phase, which is further determined by the Martensite start and finish temperature, Ms & Mf, of each alloy and the as-quenched microstructure. During heat treatment of all three classes, the solution austenitisation heat cycle is the first step, followed by rapid cooling.
The classes of high-strength Precipitation-Hardening Stainless Steels are:
Individual Alloys and Welding Information
This martensitic precipitation-hardening stainless alloy is capable of high strength (exceeding 210,000psi UTS) and hardness along with good levels of resistance to general corrosion and stress corrosion cracking. It is uniquely strengthened by the uniform formation of very finely dispersed coherent NiAl intermetallic second phase. The exceptional mechanical properties of this alloy, even under severe environmental conditions, are obtained by the necessary tight control of the chemical composition and microstructure through specialized melting practice. In order to possess consistent through-thickness and isotropic properties, PH13-8Mo is produced with controlled low levels of residual and impurity elements.
PH13-8Mo stainless has good fabrication characteristics and can be age-hardened by a single low temperature treatment. Cold work prior to aging increases the aging, especially for lower aging temperatures. PH13-8Mo stainless has been used for valve parts, fittings, cold-headed and machined fasteners, shafts, landing gear parts, pins, lock-washers, aircraft components, nuclear reactor components and petrochemical applications requiring resistance to stress-corrosion cracking. Generally, this alloy is considered where high strength, toughness, corrosion resistance, and resistance to stress-corrosion cracking are required in steel showing minimal directionality in properties. (minimum anisotropy)
PH13-8Mo, vacuum melted, MC-GRADE® high purity welding filler metal, produced to AMS5840, is widely acclaimed to be the most suitable welding alloy for use with the matching base alloy for demanding applications.
The perceptive application of MC-GRADE® processing of advanced engineering welding filler metals has enabled welding engineers to successfully fabricate many high grade materials. Underlying all of this work is the consistent control of trace elements and interstitial gases.
17-4PH stainless steel is the most widely used of all the precipitation-hardening stainless alloys, combining high strength and post-heat treatment hardness with excellent corrosion resistance that, in tests, proved superior to standard grades such as 420, 431 and 410. The corrosion resistance of 17-4PH is comparable with that of grade 304.
A higher chromium content-copper-containing stainless steel, standardised as UNS S17400, 17-4PH is highly resistant to stress corrosion, has good machinability, with short-term, low temperature heat-treatments that minimise warpage and scaling, and is considered to be more cost effective than many high nickel non-ferrous alloys.
Alloy 17-4PH martensitic precipitation-hardening stainless steel derives its high strength from the formation of copper-rich second phase coherent precipitates during the ageing cycle. The grade combines high strength, hardness (up to 320ºC) good toughness in both base metal and welds.
Due to the high strength of 17-4PH, its main use is in aerospace and other high technology industries such as offshore, gears, valves, turbine blades, nuclear waste casks, hovercraft, moulding dies and shafts, as well as having been tested in the petrochemical, petroleum, paper, dairy and food processing industries, and in such applications as boat shafting. The unique combination of properties within 17-4PH makes this alloy an effective solution to many design and production problems. The typical chemistry specification for 17-4PH to UNS17400, indicates the close control over residual elements required for consistent levels of mechanical properties.
Mechanical properties can be optimized with heat treatment. Very high yield strength up to 1100-1300 MPa (160-190 ksi) can be achieved.
This grade should not be used at temperatures above 572ºF (300ºC) or at very low temperatures. it has adequate resistance to atmospheric corrosion or in diluted acids or salts where its corrosion resistance is equivalent to grade 304 or 430.
U.S. Welding Corp. manufactures widely used, high-grade matching filler metal for 17-4PH to AMS 5825 and AMS5803 in MC-GRADE® and HQ-GRADE™. Both of these welding materials are universally used in code welding procedures worldwide. Large volumes of AMS 5803 have been used to fabricate military hovercraft. Low oxygen content (<80ppm) is the measure of excellence for these filler metals produced by USWC. [/av_tab] [av_tab title='15-5PH ' icon_select='yes' icon='ue855' font='entypo-fontello'] 15-5PH Martensitic Precipitation-Hardening Stainless Steel Alloy (S15500) is a variant of the older 17-4PH (S17400) chromium-nickel-copper precipitation hardening stainless steel. Both alloys exhibit high strength and moderate corrosion resistance. High strength is maintained to approximately (320ºC). The 15-5PH alloy was designed to have greater toughness than 17-4PH, especially in the through-thickness (short transverse) direction. This improved toughness is achieved by reduced delta ferrite content and control of inclusion size and shape.
The composition and processing of 15-5PH alloy is carefully controlled to minimize its content of delta ferrite, which is present in the 17-4PH stainless steel material. Inclusion control is achieved by consumable electrode remelting using the electro-slag remelting (ESR) process. The 15-5PH alloy is martensitic in structure in the annealed condition and is further strengthened by a relatively low temperature heat treatment which precipitates a finely dispersed coherent copper-containing phase in the alloy.
Like the 17-4PH alloy, the 15-5PH stainless steel alloy requires only a simple heat treatment; a one step process conducted at a temperature in the range (480ºC) to (620ºC) depending on the combination of strength and toughness desired. A wide range of properties can be produced by this one step heat treatment. Heat treatment in the (480ºC) range produces highest strength, although slightly less than those of semi-austenitic alloys like S17700 (17-7PH) or S15700 (15-7PH). the latter precipitation hardening alloys generally require more steps to complete heat treatment. The 15-5PH alloy is generally better-suited for plate applications than are the semi austenitic alloys.
15-5PH is used in applications requiring better corrosion resistance and improved transverse properties (critical isotropy) compared to other similar martensitic grades. Strength and toughness desired can be altered by temperate range in the heat treatment process.
USWC produces MC-GRADE® 15-5PH filler metal which is a necessary high quality material for welding 15-5PH. As with other filler metals in this series, residual and impurity elements and oxygen content are maintained at a consistent low level in order to produce toughness at high strength level. Filler metal must have an ultra clean wire surface for application to this special group of high performance materials.
17-7PH is a precipitation-hardening stainless steel that provides high strength and hardness, excellent fatigue properties, good corrosion resistance and minimum distortion upon heat treatment. It is easily formed in the annealed austenitic condition, and then hardened to high strength levels by heat treatments to conditions RH 950 and TH 1050.
The exceptionally high strength of Condition CH 900 offers many advantages where limited ductility and workability are permissible. In its heat-treated condition, this alloy provides exceptional mechanical properties at temperatures up to (480C). Its corrosion resistance in both heat-treated conditions TH 1050 and RH 950 is superior to that of the hardenable chromium types. In some environments, corrosion resistance approximates to that of the austenitic chromium nickel stainless steels. In Condition CH 900, it’s general corrosion resistance is comparable to that of Types 302 and 304 stainless steels. Fabricating practices recommended for other chromium-nickel stainless steels can be used for this material.
Since aluminium is added as an active precipitating component for NiAl second phase formation, GTAW is necessary to protect this element from oxidation during welding and to obtain acceptable weldment properties. Otherwise the alloy can be considered to have poor weldability compared to 17-4PH for instance. Post-weld heat treatments are required for strength restoration where design properties are required.
PH15-7Mo is a semi-austenitic precipitation-hardening stainless steel that provides high strength and hardness, good corrosion resistance, and minimum distortion on heat treatment. It is easily formed in the annealed condition and develops an effective balance of properties by simple heat treatments. For application requiring exceptionally high strength, cold-reduced PH15-7Mo Stainless Steel in Condition CH 900 is particularly useful for applications permitting limited ductility and workability. This alloy is particularly beneficial for a wide range of applications that include retaining rings, springs, diaphragms, aircraft bulkheads, welded and brazed honeycomb fittings and other aircraft components requiring high strength at moderately elevated temperatures.
The strengthening mechanism is by a degree of cold-working combined with precipitation of finely dispersed coherent nial second phase within the fine grained, lath martensite matrix.
The precipitation-hardening class of stainless steels is generally considered to be weldable by the common fusion and resistance techniques. Special consideration is required to achieve optimum mechanical properties by considering the best heat-treated conditions in which to weld, and which heat treatments should follow welding. This particular alloy is generally considered to have poorer weldability compared to the most common alloy of this stainless class, 17-4PH Stainless Steel. A major difference is the high Al content of this alloy, which degrades penetration and enhances weld slag formation during arc welding. Also, the austenite conditioning and precipitation hardening heat treatments are both required after welding to achieve high strength levels. When weld filler is needed, USWC MC-GRADE® PH15-7Mo to AMS 5812 is specified. This high purity, low oxygen version is an essential aspect for welding engineering.
Alloy AM 350 is a chromium-nickel-molybdenum-carbon/nitrogen precipitation-hardening stainless steel which can be hardened by martensitic transformation and/or precipitation hardening. It has been used for gas turbine compressor components such as blades, discs, rotors, shafts and similar parts where high strength was required at room and intermediate temperatures. Depending upon the heat treatment, alloy AM 350 may have an austenitic structure for best formability or a martensitic structure with strengths comparable to those of martensitic steels. The alloy normally contains about 5 to 10% delta ferrite. The corrosion resistance of alloy AM 350 approaches that of the chromium-nickel austenitic stainless steel.
The precipitation-hardening reaction in the alloy is independent of the presence of aluminium or titanium, which are reactive elements. For this reason, AM 350 alloy welds much more readily than other precipitation hardenable stainless steels. Welding procedures are designed to protect against the loss of chromium or changes in carbon content.
As-welded AM 350 is substantially austenitic and will exhibit mechanical properties which are roughly equivalent to annealed (Condition H) material. The ductility of the weld eliminates the need for preheating and post-weld annealing procedures required for the conventional and age-hardenable martensitic alloys. To produce high-strength welds, however, full post-weld heat treatment (solution annealing plus austenitic conditioning, transformation and precipitation hardening) is necessary.
AM 350 precipitation-hardening semi-austenitic stainless steel can be satisfactorily welded using USWC AMS 5774 MC-GRADE®, high purity matching analysis welding filler metal. When designing the weld joint, care should be exercised to avoid stress concentrators, such as sharp corners, threads and partial-penetration welds. Preheating is not required to prevent cracking. If possible, the weldment should be annealed after welding to provide the optimum combination of strength, ductility, and corrosion resistance. The alloy must be treated at (930ºC) before hardening by sub-zero cooling and tempering.
The precipitation hardening reaction in the alloy is independent of the presence of aluminium or titanium, which are reactive elements. For this reason, AM 350 alloy welds much more readily than other precipitation hardenable stainless steels. Welding procedures are designed to protect against the loss of chromium or changes in carbon content.
Alloy AM 355 is a semi-austenitic chromium-nickel-molybdenum-carbon precipitation-hardening stainless steel which has been used for gas turbine compressor components such as blades, rotors and shafts. Alloy AM 355 has good resistance to atmospheric corrosion and to a number of other mild chemical environments. Strengthening mechanisms are applied through carbide formation and lattice phase changes.
This alloy is a heat treatable austenitic/martensitic alloy (depending on heat treatment) that brings together the strength of heat treatable martensitic grades with the corrosion resistance of some of the chromium nickel austenitic grades. AM 355 is similar to AM 350 with the exception of a slight increase in Carbon and decrease in Chromium, which changes the Ms &Mf as well as the carbide content and matrix phase balance.
While preheating the weld piece is not required, it is recommended that the material be annealed after welding to re-attain maximum properties. For best strength at elevated temperature and sound weldment properties, U.S. Welding MC-GRADE® matching welding filler metal, AMS 5780 is selected, using the GTAW process.