Publications and Product Manual

Weldability is a term used to describe the relative ease or difficulty with which a metal or alloy can be welded.

The better the weldability, the easier it is to weld. However, weldability is a complicated property, as it encompasses the metallurgical compatibility of the metal or alloy with a specific welding process, its ability to be welded with mechanical soundness, and the capacity of the resulting weld to perform satisfactorily under the intended service conditions.

Before attempting to weld any material, it is essential to know how easy it is to weld and to be aware of any problems that might arise. One of the main problems likely to be encountered when welding carbon and alloy steels is hydrogen cracking. For hydrogen cracking to occur, it is necessary to have a supply of hydrogen to the weld and a heat affected zone (HAZ), a susceptible hardened microstructure, and tensile stress. If any one of these three components is eliminated, then hydrogen cracking will not happen. Solidification cracking and lamellar tearing are other potential problems associated with welding steel.

The main problem when welding steel is hardenability. As long as the steel contains sufficient carbon when it is cooled rapidly from high temperature, a phase transformation takes place. The phase transformation from austenite to martensite causes the material to harden and become brittle. It is then liable tocrack on cooling, due to restraint, or later under the action of hydrogen.

Afrox has a range of pipe welding electrodes that provides the best possible arc stability, penetration and wash-in.

These are ideal for welding in all positions and produces an X-ray quality weld with light slag that is easy to remove. They can be used to weld a wide range and for the root pass on material up to X-80. They feature enhanced weldability and increased mechanical properties.

Low alloy steels differ from plain carbon steels in that their characteristic properties are due to elements other than carbon and manganese, e.g. chromium, nickel, molybdenum, etc.

From the above statement, it is obvious that a wide range of steels, having different compositions and heat treatments, are available. Afrox welding consumables are available for welding three types of low alloy steels, with widely varying uses, i.e. creep resisting steels, high tensile steels and steels for use at low temperatures. The welding consumables, steels and procedures for welding these steel types are discussed separately.

Steels for Elevated Temperature Service

Creep is a property of great importance in materials used for elevated temperature applications. Creep is defined as the plastic deformation of steel occurring at an elevated temperature under constant load. Creep is a time dependent failure and occurs at stresses below the yield strength for the particular temperature to which the material is subjected.

Creep occurs in three stages:

• Primary creep (transient stage) – In this stage the creep rate is initially high and gradually decreases due to the effect of work hardening.

• Secondary creep (steady state creep) – The stage in which deformation continues at a constant rate, which results from a balance being maintained between the competing processes of work hardening and recovery.

• Tertiary creep – If the stress is sufficiently high in this stage, the creep rate accelerates until fracture occurs. For practical purposes the resistance to creep is expressed by:

• Creep strength - The stress which, at a given temperature and after a given time, causes failure.

• Creep limit - The stress which, at a given temperature and after a given time, causes a certain amount of deformation, e.g. 1%.

Creep Resisting Steels

When materials are subject to elevated temperatures, the following properties are of major importance – the resistance of the materials to oxidation and the maintenance of an adequate level of tensile strength and creep resistance. Furthermore, the steels must be capable of operating at these elevated temperatures for an indefinite period. It is a well-known fact that chromium increases the strength and oxidation resistance of steel while molybdenum increases the red hardness of steel and its elevated temperature tensile properties. It is not surprising, therefore, that these two elements are the major alloying additions to these steel types. A wide range of creep resisting steels containing between 0,5 and 1% molybdenum and up to 12% chromium have been developed for use in the power generation and petroleum refining industries.

While the addition of chromium and molybdenum improves the elevated temperature properties of the steel, they also significantly increase the hardenability of the steel. It is therefore of the utmost importance that these steels be preheated prior to welding and maintained at the preheat temperature for the duration of welding. Immediately after welding, the fabrications should be stress relieved before cooling below the preheat temperatures.

Stainless steels is a group of high alloy steels, which contain at least 12% chromium. In general, these steels are alloyed with a number of other elements which make them resistant to a variety of different environments.

In addition, these elements modify the microstructure of the alloy which in turn has a distinct influence on their mechanical properties and weldability. Stainless steels can be broadly classified into five groups as detailed below:

• • Austenitic stainless steels which contain 12 - 27% chromium and 7 - 25% nickel
• • Ferritic stainless steels which contain 12 - 30% chromium with a carbon content below 0,1%
• • Martensitic stainless steels which have a chromium content of between 12 and 18% with 0,15 - 0,30% carbon
• • Ferritic-austenitic stainless steels which contain 18 - 25% chromium, 3 - 5% nickel and up to 3% molybdenMartensitic-austenitic steels which have 13 - 16% chromium, 5 - 6% nickel and 1 - 2% molybdenum. The first four of these groups will be discussed in detail below

Austenitic Stainless Steels

This is by far the largest and most important group in the stainless steel range. These steels, which exhibit a high level of weldability, are available in a wide range of compositions such as the 19/9 AISI 304 types, 25/20 AISI 310 types and 19/12/2 AISI 316 types, which are used for general stainless steel fabrications, elevated temperature applications and resistance to pitting corrosion respectively. As the name implies, the microstructure of austenitic stainless steel consists entirely of fine grains of austenite in the wrought condition. When subjected to welding, however, a secondary ferrite phase is formed on the austenite grain boundaries, in the heat affected zone and in the weld metal. The extent of the formation of this secondary phase is dependent on the composition of the steel or filler material and the heat input during welding.

While delta ferrite formation can have negative effects on the resistance to corrosion and formation of sigma phase at operating temperatures between 500°C and 900°C, delta ferrite in weld metal is necessary to overcome the possibility of hot cracking.

In general, austenitic welding consumables deposit a weldment containing 4 - 12% delta ferrite. For special applications, i.e. when dissimilar steels are welded under conditions of high restraint, austenitic consumables having weld metal delta ferrite contents as high as 40%, may be required. The delta ferrite can be calculated using the procedure given at the end of this section with the aid of the Schaeffler diagram.

The carbon content of austenitic stainless steels is kept at very low levels to overcome any possibility of carbide precipitation, where chromium combines with available carbon in the vicinity of the grain boundaries to produce an area depleted in chromium, which thus becomes susceptible to intergranular corrosion.

The titanium and niobium stabilised AISI 321 and 347 steels together with ELC (extra low carbon) grades are available to further overcome this problem.

Ferritic Stainless Steels

These steels which contain 12 - 30% chromium with carbon content below 0,10% do not exhibit the good weldability of the austenitic types. The steels, which become fully ferritic at high temperatures and undergo rapid grain growth, lead to brittle heat affected zones in the fabricated product. No refinement of this coarse structure is possible without cold working and recrystallisation. In addition, austenite formed at elevated temperatures may form martensite upon transformation, which can cause cracking problems. The brittleness and poor ductility of these materials have limited their applications in the as welded condition.

Ferritic stainless steels are also subject to intergranular corrosion as a result of chromium depletion from carbide precipitation. Titanium and niobium stabilised ferritic steels and steels with extra low interstitials (i.e. C, N) are available to overcome this problem.

As this material has a coefficient of expansion lower than that of carbon manganese steels, warpage and distortion during welding is considerably less. They are magnetic, however, and therefore subject to magnetic arc blow. Ferritic stainless steels cannot be hardened by conventional heat treatment processes.

Martensitic Stainless Steels

Martensitic stainless steels contain between 12 - 18% chromium with 0,15 - 0,30% carbon. As a result of their composition, these steels are capable of air hardening and thus special precautions should be taken during welding to overcome possible cracking. Cold cracking, as a result of hydrogen, which is experienced with alloy steels, can also occur in martensitic stainless steels and thus hydrogen-controlled consumables must be used.

Martensitic steels, because of their lower chromium content and responsiveness to heat treatment, have limited applications for corrosion resistance but are successfully used where their high strength and increased hardness can be utilised, e.g. turbine blades, cutlery, shafts, etc.

As in the case of ferritic stainless steels, the martensitic types have a lower coefficient of expansion than mild steels and are magnetic.

Steels containing carbon in excess of 0,25%, chromium and molybdenum over 1,5% and manganese over 1,5% exhibit increased strength and hardenability and decreased weldability.

Additional elements such as vanadium, silicon, nickel, boron, niobium and titanium also influence hardenability and weldability. Steels of increased hardenability tend to form brittle microstructures in the heat affected zone, which may result in cracking. Steels featuring reduced weldability are commonly referred to as ‘problem steels’ as a result of the problem areas that are directly caused by shrinkage stresses, rapid cooling rates and the presence of hydrogen.

Electrodes for welding problem steels are chromium nickel austenitic types containing delta ferrite in the range of 10–80%. The weld metal is insensitive to hot cracking above 1 200°C. At ambient temperatures, the weld metal is strong and tough and is capable of withstanding heavy impact and shock loading in service.

Problem steels fall into two categories, i.e. ferritic types which require preheat and austenitic steels such as 11–14% manganese steels, which require minimum heat input.

When hardenable ferritic steel types are to be welded, reference should be made to the section on mild and medium tensile steels for the calculation of the carbon equivalent and preheat temperatures.

Problem steel electrodes are suitable for welding combinations of dissimilar steels such as chromium, molybdenum, creep resistant steels and stainless steels to mild and low alloy steels. Care should be taken when welding such combinations to ensure that excessive dilution between the base and weld metal does not occur.

Hard surfacing is the deposition of a special alloy material on a metallic part, by various welding processes, to obtain more desirable wear properties and/or dimensions.

The properties usually sought are greater resistance to wear from abrasion, impact, adhesion (metal-to-metal), heat, corrosion or any combination of these factors.

A wide range of surfacing alloys is available to fit the need of practically any metal part. Some alloys are very hard, others are softer with hard abrasion resistant particles dispersed throughout. Certain alloys are designed to build a part up to a required dimension, while others are designed to be a final overlay that protects the work surface.

Cast irons, like steels, are essentially alloys of iron and carbon, but whereas the carbon content of steel is limited to a maximum of 2%, cast irons generally contain more than 2% carbon.

Cast iron components contain more carbon and silicon than normal steel. With carbon content of about 2%-4%. In the four major types of cast iron (classified according to their graphite morphologies), gray cast iron is the most common. The higher carbon content makes cast irons be less ductile and more susceptible to cracking.

While many in the welding community consider cast iron to be difficult to weld, Afrox has circumvented this challenge by offering a suitable consumable and deposition technique for a particular cast iron application. Typical cast iron applications include machine bases, pump bodies, engine blocks, gears and transmission housings etc.

To facilitate a better understanding of these materials, they can be divided into five groups, based on composition and metallurgical structure: white cast iron, malleable cast iron, grey cast iron, ductile cast iron and alloy cast iron.

White Cast Iron

White cast iron derives its name from the white, crystalline crack surface observed when a casting fractures.

Malleable Cast Iron

Malleable cast iron is produced by heat treating white cast iron of a suitable composition. Iron carbide can decompose into iron and carbon under certain conditions.

Ferritic Malleable Cast Iron

At room temperature, the microstructure therefore consists of temper carbon nodules in a ferrite matrix, generally known as ferritic malleable cast iron.

Perlitic Malleable Cast Iron

If full graphitisation is prevented and a controlled amount of carbon remains in the iron during cooling, finely distributed iron carbide plates nucleate in the iron at lower temperatures.

Grey Cast Iron

Grey cast iron is one of the most widely used casting alloys and typically contains between 2,5% and 4% carbon and between 1% and 3% silicon.

Copper is a metal with some very important properties, the main ones being its high electrical conductivity, its high thermal conductivity, its excellent resistance to corrosion, and its ease of fabrication, either hot or cold.

Copper is also ductile and malleable and has a relatively low melting point at just over 1 080°C

The three basic commercial grades of copper that are available:

• • Tough pitch copper, containing up to 0,1% oxygen
• • Phosphorous deoxidized (PDO) copper, containing up to0,04% phosphorous
• • Oxygen-free copper, containing no deoxidants.

The phosphorous deoxidised grade was originally developed to overcome problems encountered when flame welding tough pitch copper. It is now the standard commercial weldable grade used for pressure vessels and radiators. Oxygen-free grades have significantly higher electrical conductivity than oxygen-containing grades and are therefore used widely as electrical conductors.

Aluminium is a light, ductile, readily worked metal, with good thermal and electrical properties. It has a tenacious oxide film on the surface that gives it good corrosion resistance. It is also the most abundant metal on earth.

Aluminium alloys may be sub-divided into two main groups – cast alloys and wrought alloys. Wrought materials also come in a wide variety of product forms.

Wrought alloys are further sub-divided into heat treatable and non-heat treatable alloys.

Heat treatable alloys are based on aluminium-copper, aluminium silicon-magnesium and aluminium-zinc-magnesium alloy systems. They can develop high strength by solution treatment followed by age hardening at elevated temperatures.

Non-heat treatable alloys include pure aluminium, and those based on aluminium-manganese, aluminium-silicon, and aluminium magnesium. They can be strengthened only by cold work.

Nickel-based alloys have nickel as their main constituent, generally making up over 50% of these types of alloys.

They are primarily used for their corrosion properties, but are also used for their heat resistance, low expansion characteristics and electrical resistance.

The most important group of general-purpose nickel alloys are the Inconel types which are based on the heat resistant alloy 600 made of nickel, chrome and iron. These alloys are used in applications from cryogenic processes at -196ºC to elevated temperatures of up to 1 000ºC. They are also used in power generation for steam turbine power plants, aircraft gas turbines, nuclear power plants, furnaces as well as in the chemical and petrochemical industries.

Monel 400 is an alloy made from nickel and copper, used in marine and offshore environments for the fabrication of heat exchangers, evaporators, piping and vessels as well as in the chemical, petrochemical, and power generation industries.

These alloys are known by many other proprietary alloy names in the industries in which they are used, such as Inconel, Monel and Incoloy from Special Metals, Nicrofer and Nicorros from Krupp VDM, Pyromet from Carpenter Alloys and Hastelloy from Haynes International Inc. Generally, these alloys are readily joined by most welding processes.

Nickel alloys can be joined by all the common types of welding process such as Manual Metal Arc (MMA), Metal Inert Gas (MIG), Tungsten Inert Gas (TIG) and Submerged Arc Welding (SAW), but not by the forge welding or oxy-acetylene processes. For the majority of applications on wrought nickel alloys, no preheat or post weld heat treatment will be required. In certain special cases, a post weld heat treatment may be required for stress relief of a fabricated structure or to avoid age hardening and stress corrosion cracking problems in acid or caustic environments. Nickel and nickel alloys can however be susceptible to embrittlement by low melting point elements such as sulphur, lead and phosphorous. These elements can occur in grease, paint, oil crayons, inks, cutting fluids, shop dirt or processing chemicals. It is therefore important that components to be welded are completely free of these contaminants before welding begins.

An alloy is normally selected for its melting and flow characteristics.

The easiest filler materials to use are the high silver, free-flowing alloys, because of their low melting temperatures and narrow melting ranges. The higher the brazing temperature and the wider the melting range of the alloy, the more difficult the brazing operation will be.

Pre-Cleaning

It is important that the mating surfaces of the components to be brazed are free from oil, grease and any surface oxide layer prior to joining. Most engineering components require nothing more than degreasing before assembly.