Low Alloy Steel in Aerospace

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Authors: Surendra M. Vaidya, Executive VP & Business Head, Godrej Aerospace. Hitesh A. Narsia, Asst. Manager – Welding, Godrej Aerospace

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Vikas Engine 25CD4S

 

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Godrej Aerospace, the Business unit of the 120 year old Godrej & Boyce Mfg. Co. Ltd. has played a key role in building the Liquid engines used in the rockets of ISRO including the Polar Satellite Launch Vehicle (PSLV) & Geosynchronous Launch Vehicle (GSLV). Starting in 1985, GA’s manufacturing of critical components for ISRO’s satellite marked the beginning of Godrej’s entry into the complex Aerospace manufacturing segment. The Geosynchronous Satellite Launch Vehicle (GSLV-Mk III), The latest of the launch vehicles from ISRO, launched by ISRO in June 2017, from its spaceport at Sriharikota in Andhra Pradesh is a three-stage vehicle with two solid motor strap-ons (S200), a liquid propellant core stage (L110) and a cryogenic stage (C25) capable of launching 4 ton class of satellites to Geosynchronous Transfer orbit (GTO). Godrej Aerospace contributed to the L110 stage and the C25 stage by manufacturing the Vikas L110 engine and The Thrust Chamber for the C25 stage Cryogenic engine. GA currently manufactures for the Space, Defence and Aircraft segments where Low Alloy Steel material is used. The hardware’s related to Low Alloy Steel material are Vikas Engine used in Space Launching Vehicle and the Rocket Motor Casing used in DRDO Project.

The low-alloy steels constitutes a category of ferrous materials that exhibits mechanical properties that are superior to plain carbon steels as the result of addition of alloying elements such as nickel, chromium, and molybdenum. Total alloy content ranges from 2% up to levels just below that of stainless steels, which contain a minimum of 10% Cr. For many low-alloy steels, the primary function of the alloying elements is to increase Strength and Fracture Toughness in order to optimise mechanical properties and toughness after heat treatment. As with steels in general, low-alloy steels can be classified according to:

  • Heat treatment, such as quenched and tempered, normalized and tempered, annealed.
  • Chemical composition, such as nickel steels, nickel-chromium steels, molybdenum steels, chromium-molybdenum steels.

Low Alloy Steels used in Aerospace Industry

Low Alloy Steel, because of the wide variety of chemical compositions, some steels are used in more than one heat-treated condition, some overlap exists among the alloy steel classifications. In Aerospace Industry, mainly four groups of alloy steels are addressed:

  • High Strength, Low Alloy Steels
  • Treatable Low Alloy Steels
  • Low temperature use steel (Nickel Steel)
  • Quenched and Tempered Low Alloy Steels

High-Strength, Low-Alloy (HSLA) Steels – HSLA are designed to provided higher strengths than those of carbon steels, generally with minimum yield strengths of 275–550MPa. Besides, manganese (up to about 1.5%) and silicon (up to about 0.7%), as in carbon steels, HSLA steels often contain very small amounts of niobium (up to about 0.05%), vanadium (up to about0.1%), and titanium (up to about 0.07%) to ensure both grain refinement and precipitation hardening. As such, they are also called micro alloyed steels. Typically, the maximum carbon content is less than 0.2% and the total alloy content is less than 2%. Niobium (Nb), vanadium (V), and titanium (Ti) are strong carbide and nitride formers. This makes it most effective in limiting the extent of grain growth in welding. The higher the heat input during welding, the more likely the carbide and nitride particles will dissolve and lose their effectiveness as grain growth inhibitors. The low toughness of the coarse-grain regions of the HAZ is undesirable. The HSLA steels are usually welded in the as-rolled or the normalized condition, and the weldability of most HSLA steels is similar to that of mild steel. Since strength is often the predominant factor in the applications of HSLA steels, the filler metal is often selected on the basis of matching the strength of the base metal. Preheat and interpass temperatures required are relatively low as aerospace materials having lower thicknesses. The amount of preheating required increases with increasing carbon and alloy content and with increasing steel thickness.

The Heat-Treatable Low-Alloy (HTLA) Steels – HTLA refer to medium-carbon quenched-and tempered low-alloy steels, which typically contain up to 5% of total alloy content and 0.25–0.50% carbon and are strengthened by quenching to form martensite and tempering it to the desired strength level. The higher carbon content promotes higher hardness levels and lower toughness and hence a greater susceptibility to hydrogen cracking than the quenched and-tempered low-alloy steels. Alloys 4130, 4140, and 4340 are examples of HTLA steels. The HTLA steels are normally welded in the annealed or over tempered condition except for weld repairs, where it is usually not feasible to anneal or over temper the base metal before welding. Immediately after welding, the entire weldment is heat treated, that is, reaustenized and then quenched and tempered to the desired strength level, or at least stress relieved or tempered to avoid hydrogen cracking. The weld metal hydrogen must be maintained at very low levels; proper interpass temperatures must be maintained. In applications where the weld metal is required to respond to the same post weld heat treatment as the base metal in order to match the base metal in strength, a filler metal similar to the base metal in composition is used. HTLA Weld metal has lower strength and greater ductility than the quenched-and tempered base metal, and high shrinkage stresses during welding can result in plastic deformation of the weld metal rather than cracking of the HAZ.

Nickel Steel – The average steel in low temperature environment will have higher strength but lower elongation and toughness, thus increases the chance for brittle fracture. If the steel is needed in a low temperature environment, having superior low temperature toughness is essential. Any suitable steel for this purpose is called low temperature service steel or Nickel steel. Low Alloy Low Temperature Service Steel is formed by adding 2.5% to 3.5% of Ni in the carbon steel to enhance its low temperature toughness. Ni can strengthen ferrite matrix while lowering Ar3 (third transformation temperature) which helps with fine grain formation. In addition to the normalizing treatment during the production process of low alloy low temperature service steel, quenching and tempering are also parts of the mechanical properties improvement treatment. Nickel alloys are widely using in Cryogenic and Semi Cryogenic applications.

Quenched and Tempered Low Alloy Steels – It usually containing less than 0.25% carbon and less than 5% alloy, are strengthened primarily by quenching and tempering to produce microstructures containing martensite and bainite. The yield strength ranges from approximately 345 to 895MPa depending on the composition and heat treatment. Low carbon content is desired in such alloys for the following two reasons: (i) To minimize the hardness of the martensite and (ii) to raise the Ms (martensite start) temperature so that any martensite formed can be tempered automatically during cooling. Due to the formation of low-carbon auto-tempered martensite, both high strength and good toughness can be obtained. Alloying with Mn, Cr, Ni, and Mo ensures the harden ability of such alloys. The use of Ni also significantly increases the toughness and lowers the ductile–brittle transition temperature in these alloys. Weld metal hydrogen must be maintained at very low levels. Preheating is not required due to less thickness, but interpass temperatures required Excessive heat input can also decrease the cooling rate and produce unfavourable microstructures and properties. If the cooling rate during welding is too low, a substantial amount of ferrite forms. This can, in fact, be harmful since the ferrite phase tends to reject carbon atoms and turn its surrounding areas into high-carbonaustenite. Such high-carbon austenite can in turn transform to high-carbon martensite and bainite during cooling, thus resulting in a brittle HAZ. Therefore, the heat input and the preheating of the work piece should be limited when welding quenched-and-tempered alloy steels. To meet the requirements of both limited heat inputs and proper preheating, multiple-pass welding is often used in welding thick sections of QTLA steels. In so doing, the interpass temperature is maintained at the desired level. Multiple-pass welding with many small stringer beads improves the weld toughness as a result of the grain-refining and tempering effect of successive weld passes. The martensite in the HAZ of a weld pass is tempered by the heat resulting from deposition in subsequent passes. As a result, the overall toughness of the weld metal is enhanced.

Aerospace Standard

Aerospace is a generic term that includes commercial aircrafts, planes and helicopters, military and defense, ground equipments, and space. Producing aerospace parts leaves a long supply chain, creating a major challenge for the industry.  As this chain is so long and complicated, it should come as no surprise that Aerospace standards are so stringent with the expectations it has for maintaining manufacturers. Manufacturing sophisticated vehicles, such as airplanes or rockets understandably needs special attention throughout the entire production process. It needs to ensure all the correct revisions of any engineering documentation is recorded and used within instructions, and that performance is noted. Controlling production processes is an essential way to prove that operations have been correctly performed. This is more important than ever when it concerns processes that can’t be inspected after-the-fact. As the industry often relies upon production equipment and tooling, such as computer-controlled machinery, this too forms part of the basis for product acceptance. When this is the case, it must ensure to demonstrate the integrity of the equipment, developing a process to provide sufficient oversight of the entire process. Aerospace industries rely on rigorous quality requirements, advanced materials, mechanical components as well as. The aerospace industry has a strong certification and compliance requirement, with consequences on development cost and technology solutions.

AS9100 ensures aerospace manufacturers, being up-to-date with the quality standards is essential in order to supply parts to the industries. System Structure is complex by disciplines, segment, component, etc. by large system management combined with high precision. Data access and security is critical, especially on Defence and Space Programmes.

NADCAP ensures advancements in the aerospace industry continue, it is not only need to ensure they obtain the required certification, but keeping up to date with revisions. The standard additionally focuses on aerospace manufacturers being able to improve their quality management system between their audits, which can make the certification quite difficult to maintain. NADCAP is important to all special processes like welding, Heat Treatment, Surface Treatment, Non Destructive Testing.

Customer Approval is required to meet the design criteria depending upon the product. Different customers have different criteria depending upon the criticality of the product.  Separate Procedures, Welders and Facilities need to qualify and maintained as per the customer requirements.

Product Specific Qualification and Acceptance after Meeting specification requirements, before commencing on actual hardwares. Mock up of the same size need to perform and shall be resulting positive in all destructive and non destructive tests.

Design

Unlike any other industry but oil and gas, which also has high temperature, pressure, and corrosion requirements, aerospace materials themselves impact component design. No standard international accepted code is available for aerospace industry. Design for manufacturability (DFM) is the engineering art of designing components with a balanced approach, taking into consideration both component function and its manufacturing requirements. This approach is being applied more and more in aerospace component design because its components have to accomplish certain loads and temperature resistances, and some materials can only accommodate so much.

Handling of Aerospace Materials

The precautions required for handling material in Aerospace Industries are very important. Cleanliness is paramount. Contamination can cause porosity or cracking during welding or heat treatment may affect the mechanical properties of the alloy. Material should remain in their packing until needed and should be separated, normally by protective materials like Oil, and Anti Corrosive Black Japan to protect from the atmosphere. Storage of welding consumable should be in Hot Rooms or in inert atmosphere to avoid contamination. Lint free gloves to be used during handling of material. Transporting or shifting of material is also very important from one work station to other work station, No dent or scratch mark is acceptable and chances of rejection rate is very high due to the lower thicknesses.

Thin Foam and Plastic sheets should be use for material movement.

Dimensional Significance

Dimensional tolerances are very stringent; it is challenging to maintain ovality, symmetricity and other required dimensions. During Machining Proper Coolants are used, During Welding and Heat Treatment Appropriate Fixturing, Cooper Cooing Ring, Heat Ban Paste, Continuous water circulation is required to minimize the ovality and dimension in stability. Advanced Metrology instruments are used to check the required dimensions. Maraging steel to Low alloy steel rocket motor casing is similar to pressure vessel, but it is not static and due to its critical function, Minimum Ovality is very significant.

Machining

Hardness and ductility plays a vital role in deciding the wear characteristics of Low Alloy Steel type and its heat treatment. Significant progress has already been achieved in aerospace and defence manufacturing including dry and hard, often high-speed, machining technologies. For instance, the demand for higher productivity has resulted in the wider application of ceramic and PCBN tools with special multi-radii (wiper) geometry. Final Stage machining contains high level of skills to maintain the accurate dimensions. Machining is carried out in fixtures, at required travel speed using coolant.

Material Qualification and Testing

Qualification of the parent material and welding material is carried out as per the customer specifications. Factor of safety is very high in aerospace standards. After Welding, material must pass NDT and DT. Common used methods of the NDT are DPT (Dye Penetrant Test) and RT (Radiographic Examination). No Indication is acceptable in DP Test. Acceptance criteria in RT is very stringent. After Clearing of NDT, Destructive Tests Like, Tensile Test, Root Bend, Face Bend, Notch Toughness Test, Macro Examination of Weld, Parent, HAZ, Micro Examination at minimum 100X for Weld, Parent and HAZ carried out. For Fillet Joint Qualification, separate test is carried out by examine the Leg Length, Weld Throat and Depth of Penetration.

Weld substantiation and Readiness

Low alloy steel having greater potential for distortion and shrinkage as aerospace materials having lower thicknesses and bigger diameters. To avoid this, welding fixtures are used at the different sub-assembly levels. All Welding joints are designed as single sided joints to due to difficult configurations, accesses and avoid back chipping. Uniform tacks are welded to maintain the uniform gap and alignment between the parts having welded. GTAW process is often used for tacking. Tacks should be wire brushed or ground to clean metal. Up to 2.0 to 2.5mm Thickness square butt joint is suitable, above 2.5mm thickness; joint configuration is prepared such that heat input is minimized. All traces (Oil, Grease, Dust, Paint, etc.) of the elements must be removed. The joint area must be thoroughly clean before welding starts. The weld preparation and on adjacent area either side of the preparation, at least 50mm wide, must be degreased and cleaned. For this, uncontaminated organic solvents should be applied with lint free clothes and the area dried. The appearance of the cloths used for drying is a useful indicator of cleanliness, they should free from any residue. There is no need to pre-heat the base material before welding unless this is necessary to ensure that base material is dry. To avoid micro fissuring, interpass temperature is restricted to 150˚C.

Welding

Low alloy steels can be welded by most processes, as long as adequate precautions are taken to avoid defects. It is important to know the composition of the material, either from a mill sheet or a dedicated chemical analysis, as composition influences weldability significantly. With increasing carbon or alloy content, low alloy steels generally become more difficult to weld as the heat affected zone hardness increases. In Aerospace applications, defect Free and Lower Heat Input welding processes are essential. To meet these requirements, GTAW, GMAW, EBW, FSW and LBW in special cases are used. Aerospace welding depends largely on skills and equipments available, although new project may justify new equipments and special training. It is highly desirable that welders are given a period of familiarization with the materials and the technique use to handle it. Clean environment, Humidity Level below 50% is mandatory. Welding position is possible in all positions, but it is desirable to used down-hand, which allows higher deposition rates with lower heat inputs. It is worth the efforts of manipulating the sub-assemblies for down hand welding, rather than attempt to operation in less favourable position.  String bead technique is preferred. Long arc is avoided to chances of porosities. Welding Down slope is kept on higher side and minimum 20 to 25 mm overlap length is required to minimise the chances of crater cracks. Most common used shielding and purging gas for welding of Similar or dissimilar welding is Ultrapure Argon Gas. (Purity 99.9997%). The general guideline for welding conditions is to avoid higher levels of heat input. The heat input should be restricted between 0.5 to 1.5 KJ/mm.

Conventional Welding Process

GTAW/TIG (Gas Tungsten Arc Welding) process can give very high quality welds in complex joints. Welding Heat Input in GTAW depends upon the type of joint and welding sequence. Separate control of heat input via the arc and filler material addition gives GTAW, a degree of flexibility, which is an advantage when welding of difficult shape joints. Welding is carried out in opposite segments to control the distortion and heat input. Welding Down slope is kept on higher side and minimum 20 to 25 mm overlap length is required to minimise the chances of crater cracks. The composition of the filler metal can be substantially different from that of the base metal, depending on the strength level of the weld metal required. Most of the aerospace industries are changing their welding process from manual mode to semi auto or fully auto mode to avoid distortion due to heat input.

GMAW/MIG (Gas Metal Arc Welding) welding process is faster process using continuous wire feeding and can be closely controlled with modern sophisticated equipments. Heat Input in GMAW depends on the mode of the metal transfer. For thinner sections, dip or short circuiting transfer arc is used. Pulsed arc transfer is a more advanced technique in which metal transfer is closely controlled, providing a combination of overall low heat input and adequate fusion of base material. More advanced synergic welding power sources control the detachment of the droplet from the wire effectively while reducing the number of variables to be set by the welder. Most preferred shielding gas is Argon or Mixture of Argon-Helium. Wire spool should be stored in dry condition to prevent contamination

Specialized Welding Process

Friction Stir Welding (FSW) – FSW does not involve bulk melting of the components that are joined. This has inspired attempts to exploit it for joining materials which differ in properties, chemical composition or structure, and where fusion can lead to detrimental reactions. The purpose of this special issue of Science and Technology of Welding and Joining was to assess the status of friction stir welding of dissimilar alloys and to identify the opportunities and challenges for the future.

Electron Beam Welding (EBW) – Due to special features of EBW, e.g., high energy density and accurately controllable beam size and location, in many cases it has proven to be an efficient way of joining dissimilar metals. Since EBW is a fusion-welding process, metallurgical phenomena associated with fusion still exist and cause difficulties. However, these are often minor as compared to those in conventional are welding

Laser Beam Welding (LBW) – Like electron beam welding (EBW), laser beam welding has high power density (on the order of 1 MW/cm2) resulting in small heat-affected zones and high heating and cooling rates. LBW is a versatile process, capable of welding of LAS and Dissimilar Joints. A derivative of LBW, laser-hybrid welding, combines the laser of LBW with an arc welding method such as gas metal arc welding. This combination allows for greater positioning flexibility, since GMAW supplies molten metal to fill the joint, and due to the use of a laser, increases the welding speed over what is normally possible with GMAW. Weld quality tends to be higher as well, since the potential for undercutting is reduced

Dissimilar Welding – In aerospace and defence sector for the space launching vehicles and rocket motors, dissimilar materials are used. In Vikas Engine 25CD4S (High Strength Low Alloy Steel) is welded with Z8CNDT (Equivalent to SS316L) and KC20WN (Satellite, Cobalt Base Alloy). Apart from cost, 25CD4S Material is used to maintain higher dimensional stability, abundant shock resistance, vibration resistance as compared to Satellite and SS material. In defence sector, in Motor Casings, Maraging steel is widely used with SAE 4130 (Heat Treatable Alloy Steel). The dissimilar materials welded with Low Alloy steels having comparatively higher strength, Filler wire of the higher strength material are being used. Crater crack welding defects are common in dissimilar alloys and these are precisely controlled by advanced welding techniques. Since Heat Input is a critical factor in low alloy steel as compare to maraging and Austenitic Grade materials, the additional cares are taken during welding and heat treatment by providing copper cooling ring, Heat Ban Paste on LAS HAZ Area. Purpose of using SAE4130 Steel with Maraging steel is mainly case design is usually governed by the combination of motor and vehicle requirements. The main purpose of the motor casing is to store the propellant. As soon as the propellant is ignited and rocket starts moving, a high amount of pressure and temperature. As the mass of the SAE4130 material is twice of the MDN250 material, SAE4130 material is used only in non pressure and firing parts.

Low Alloy Steel in Aerospace

 

Visual and NDT

After Welding, Visual inspection at 10X Magnification is carried out, welding dimensions are maintained as per the specification.  After that Dye Penetrant and FPI/MPI Test is carried out. No indication is acceptable in these examinations. After satisfactory completion of Visual and Penetrant Testing, Radiographic Examination is carried out. IQI used is much important to indicate the overall sensitivity of the technique to reveal the least discontinuity. As the name implies, an IQI is an indicator of the quality of the radiographic image. An IQI is a device made from the same material as the test specimen. It is placed on the hardware joint in a position where its image will be recorded on the radiograph. Interpretation of RT Films also plays a vital role and proper decision on weld rework has to take. Involved customers are interpreting RT Films on daily basis to ensure the quality and internal interpretation. Factor of Safety in Aerospace Industry is very marginal. Frequent rework of the hardware can directly reject the hardware.

Post Weld Heat Treatment

For Heat Treatable low alloy steel, the weldment is stress relieved or tempered after welding. The weldment of heat-treatable alloy steels is to be heated for stress-relief heat treatment. During the stress-relief heat treatment, martensite is tempered and, therefore, the weldment can be cooled to room temperature without danger of cracking. It will develop the strength and toughness and capable of attaining optimize the microstructure and properties of the weldment. In cases where heat-treatable low alloy steels cannot be post weld heat treated and must be welded in the quenched-and-tempered condition. So PWHT is generally not recommended for High Strength Low Alloy and Quenched and Tempered Steels. Aim of the PWHT is to modify the grain sizes to provide notch toughness and strength the PWHT is to modify the grain sizes to provide notch toughness and strength.

Comparison of Low Alloy Steel with commonly used aerospace materials

Properties LAS Al Alloy Ti Alloy Ni Alloy Fibre/ Composites
Cost Medium Low Expensive Expensive Expensive
Weight Heavy Low Medium Heavy V. Light
Stiffness V. High Medium Medium Medium High
Yield Strength V. High Medium Medium Medium High
Fracture Toughness Low Low High Medium Low
Fatigue Toughness Medium Medium High Medium High
Corrosion Resistance Medium Medium High High V. High
Creep Strength High Low High V. High Low
Ease of Recycle High High Medium Medium V. Low
Dissimilar Welding of MDN250 Maraging
Dissimilar Welding of MDN250 Maraging Steel to SAE 4130 Steel using Heat Ban Paste and Copper Cooling Ring

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