WIS5 Weldability
WIS5 Weldability
WIS5 Weldability
Weldability of Steels
By
Mohd Faisal Yusof
World Centre for Materials Joining Technology
Classification of Steel
Steels are classified into groups as follows
Plain Carbon Steels
1. Low Carbon Steel 0.01 0.3% Carbon 2. Medium Carbon Steel 0.3 0.6% Carbon 3. High Carbon Steel 0.6 1.4% Carbon
Plain carbon steels contain only iron & carbon as main alloying elements, traces of Mn, Si, Al, S & P may also be present
Copyright 2003 TWI Ltd
The following basic foundation information on metallurgy will not form any part of your CSWIP examination. A most important function in the metallurgy of steels, is the ability of iron to dissolve carbon in solution The carbon atom is very much smaller than the iron atom and does not replace it in the atomic structure, but fits between it
Iron atoms
Carbon atoms
Iron is an element that can exist in 2 types of cubic structures, depending on the temperature. This is an important feature
Copyright 2003 TWI Ltd
Alpha iron
This structure occurs below 723 C and is body centred, or BCC in structure It can only dissolve up to 0.02% Carbon Also known as Ferrite
or BCC iron
Gamma iron
This structure occurs above the UCT in Plain Carbon Steels and is FCC in structure. It can dissolve up 2.06% Carbon Also called Austenite
or FCC iron
If steel is heated and then cooled slowly in equilibrium, then exact reverse atomic changes take place
If a steel that contains more than 0.3% Carbon is cooled quickly, then the carbon does not have time to diffuse out of solution, hence trapping the carbon in the BCC form of iron. This now distorts the cube to an irregular cube, or tetragon This supersaturated solution is called Martensite and is the hardest structure that can be produced in steels
If some steels are cooled quickly their structure looks like this
Martensite can be defined as: A supersaturated solution of carbon in BCT iron (Body Centred Tetragonal) It is the hardest structure that can be thermally produced in steels Compressed representation could appear like this
Copyright 2003 TWI Ltd
Solubility of Carbon in BCC & FCC phases of steels Ferrite: a Low carbon solubility. Maximum 0.02%
Austenite: g High carbon solubility. Maximum 2.06% Martensite: The hardest phase in steels, which is produced by rapid cooling from the Austenite phase It mainly occurs below 300 C
TTT DIAGRAM
Copyright 2003 TWI Ltd
Diagram showing the Relationship between Carbon Content, Mechanical Properties, Microstructure and Uses of Plain Carbon Steels in the Normalised Condition
Copyright 2003 TWI Ltd
Classification of Steel
An Alloy steel is one that contains more than Iron & Carbon as a main alloying elements Alloy steels are divided into 2 groups
1. Low Alloy Steels < 7% extra alloying elements
Solid solution
(a)
substitutional
(b)
interstitial
Molybdenum: Effects harden ability, and has high creep strength at high temperatures. Steels containing molybdenum are less susceptible to temper brittleness than other alloy steels. Chromium: Widely used in stainless steels for corrosion resistance, increases hardness and strength but reduces ductility.
Nickel: Used in stainless steels, high resistance to corrosion from acids, increases strength and toughness
Copyright 2003 TWI Ltd
Increased strength: C, Si, Cu, Mn, Mo (also Nb and V; their exact effect depends on other factors also such as the rolling temperature and time, amount of carbon and nitrogen present, etc.) Hardening capacity: C, Mn, Mo, Cr, Ni, Cu Toughness: Ni, grain refinement (achieved via the presence of Nb, V, Al, Ti) Elevated Temperature Properties: Atmospheric corrosion Resistance: Cu, Ni Cr, Mo, V
The type and number of elements present in the material The temperature reached during welding and or PWHT. The cooling rate after welding and or PWHT
Cooling Rate
The cooling rate of the weld zone depends on the following factors: Weld heat : Also call arc energy, is the amount of electrical energy that is supplied to the welding arc over a given weld length ( an inch or mm)
Thickness of material
Preheating
The parent material undergoes microstructure changes due to the influence of the welding process. This area, which lies between the fusion boundary and the unaffected parent material, is called the heat affected zone (h.a.z.).
Copyright 2003 TWI Ltd
Arc energy
Amps = 200 Volts = 32 Travel speed = 240 mm/min Arc energy= Amps x volts Travel speed mm/sec X 1000
Arc Energy
High arc energy - slow cooling
Low toughness
Reduction in strength Low arc energy - fast cooling Increased hardness
Hydrogen entrapment
Lack of fusion
Copyright 2003 TWI Ltd
Carbon Equivalent
The CE of steel primarily relates to its hardenability. Higher the CE, lower the weldability
Weldability
Weldability can be defined as the ability of a material to be welded by most of the common welding processes, and
Weldability
Weldability is a function of many inter-related factors but these may be summarised as:
Access
Copyright 2003 TWI Ltd
Weldability
It is very difficult to asses weldability in absolute terms therefore it is normally assessed in relative terms
Weldability
There are many factors which affect weldabilty e.g. material type, welding parameters amps, volts travel speed, heat input.
Weldability
Other factors affecting weldabilty are welding position and welding techniques.
Weldability
Basically speaking weldabilty is the ease with which a material or materials can be welded to give an acceptable joint
Cracks
Weld Decay
Cracks
When considering any type of crack mechanism, three elements must be present for its occurrence:
Stress: stress is always present in weldments, through local expansion and contraction.
Hydrogen Cracks
Hydrogen Cracking
Hydrogen causes general embrittlment and in welds may lead directly to cracking,
Hydrogen Cracking
Characteristics Also known as hydrogen induced cold cracking , delay cracking , underbead cracking and chevron. Hydrogen is the major influence to this type of cracking. Source of hydrogen may be from moisture or hydrocarbon such as grease , paint on the parent material, damp welding fluxes or from condensation of parent material Hydrogen is absorbed by the weld pool from the arc atmosphere.
During cooling, much of this hydrogen escapes from the solidified bead by the diffusion but some also diffuses into the HAZ of the parent metal. Type of cracking is intergranular along grain boundaries or transganular Requires susceptible grain structure, stress and hydrogen and low temperature is reached. Most likely in HAZ for Carbon Manganese steel and in weld metal for HSLA steel.
Hydrogen Cracking
Micro Alloyed Steel
Carbon Manganese Steel
Hydrogen Cracking
Factors responsible: Hydrogen cracking occurs when the conditions outlined in 1 4 occur simultaneously : 1.Susceptible grain structure hardness value > 350 V.P.N That part of HAZ which experiences a high enough temperature for the parent steel to transform rapidly from ferrite to austenite and back again,produces microstructures which are usually harder and more susceptible to hydrogen embrittlement. 2.Hydrogen level - > 15 ml/100g This is inevitably present, derived from moisture in the fluxes used in welding and from other sources.
Hydrogen Cracking
3.Temperature < 200oC for any steel and < 150oC for structural steel.
The greatest risk of cracking occurs when temperatures near ambient are reached and cracking may thus take place several hours after welding has been completed ( normally after 72 hours ) 4.Stress > 50% yield strength of parent metal
These arise inevitably from thermal contractions during cooling and may be supplemented by other stresses developed as a result of rigidity in the parts to be joined.
Hydrogen Cracking
Precautions for controlling hydrogen cracking
Pre heat, removes moisture from the joint preparations, and slows down the cooling rate Ensure joint preparations are clean and free from contamination The use of a low hydrogen welding process such as TIG or MIG/MAG The use of Nickel and Austenitic filler metal
Solidification Cracks
Solidification Cracking
Characteristics
Also
known as hot cracking or center line cracking or crater cracking and liquation cracking
Solidification
cracking is intergranular type of cracking that is along the grain boundaries of the weld metal.
It
occurs during the terminal stages of solidification,when the stresses developed across the adjacent grains exceed the strength of the almost completely solidified weld metal.
Impurities
such as sulphur and phosphorous and carbon pick - up from parent metal increase the risk of cracking
High
Solidification Cracking
Occurs during weld solidification process from liquidus to solidus and at the last area to solidified. Steels with high sulphur content (low ductility at elevated temperature ) whereby produce hot shortness to the weld metal FeS form films at the grain boundaries whereby reduce the strength of the weld metal. Addition of manganese will form MnS and forms globules instead of films( FeS) Occur longitudinally down center of weld Welding process that most susceptible to this type of cracking are SAW and MIG/MAG with spray transfer due to high dilution rate.
Copyright 2003 TWI Ltd
Solidification Cracking
Solidification Cracking
Weld Centerline
Copyright 2003 TWI Ltd
Solidification Cracking
Factors responsible :
Metallurgical factors
a)
Freezing temperature range higher freezing range more susceptible to solidification cracking due to presence of FeS Primary solidification Phase Less than 5% delta ferrite Surface tension concave more susceptible than convex weld shape Grain structure of fusion zone Coarse columnar grain more susceptible especially with high energy welding process.
b) c)
d)
Mechanical factors
a) b)
Contraction stresses Thicker material more susceptible. Degree of restraint poor fit - up
World Centre for Materials Joining Technology
Solidification Cracking
Precautions for controlling solidification cracking
Use low dilution welding process
The use of high manganese and low carbon content fillers Control sulphur ,keep below 0.06% Maintain a low carbon content Minimise the amount of stress / restraint acting on the joint during welding The use of high quality parent materials, low levels of impurities Use proper joint design, use Single J instead of single V Clean joint preparations, free from oil, paints and any other sulphur containing product. Joint design selection depth to width ratios
Copyright 2003 TWI Ltd
Lamellar Tearing
Lamellar Tearing
Characteristics
Lamellar tearing has a step like appearance due
to the solid inclusions linking up under the influences of welding stresses Occurs at beneath of HAZ or near HAZ
It forms when the welding stresses act in the short transverse direction of the material (through thickness direction)
Low ductile materials containing high levels of impurities are very susceptible
World Centre for Materials Joining Technology
Lamellar Tearing
Occur only in rolled direction of the parent material Associated with restrained joints subjected to through thickness stresses on corners and tees Presence of elongated stringers such of nonmetallic inclusion such as silicates and sulfides parallel to steels rolling plane will produce poor through thickness ductility of the plate. Tearing will triggered by this such non metallic inclusion near the weld or it just outside HAZ during weld contraction.
Lamellar Tearing
Cross section
Copyright 2003 TWI Ltd
Lamellar Tearing
Susceptible joint types
Corner butt weld (single-bevel)
Lamellar Tearing
Critical area
Critical area
Critical area
Lamellar Tearing
Precautions for controlling lamellar tearing
The use of high quality parent materials, low levels of impurities ( Z type material )
The results are given as a STRA value Short Transverse Reduction in Area
Copyright 2003 TWI Ltd
Lamellar Tearing
Precautions for controlling lamellar tearing
The use of high quality parent materials, low levels of impurities ( Z type material )
Lamellar Tearing
Modifying a Tee joint to avoid lamellar tearing
Susceptible Non-susceptible Improved
Susceptible
Non-susceptible
Susceptible
Less susceptible
Gouge base metal and fill with weld metal before welding the joint
Copyright 2003 TWI Ltd
Prior buttering of the joint with a ductile layer of weld metal may avoid lamellar tearing World Centre for Materials Joining Technology
Lamellar Tearing
Modifying a corner joint to avoid lamellar tearing
Susceptible
Non-Susceptible
Lamellar Tearing
Precautions for controlling lamellar tearing
The use of high quality parent materials, low levels of impurities ( Z type material )
Minimise the amount of stress / restraint acting on the joint during welding
Place soft filler wire between the joint e.g T joint to reduce stresses during expansion and contraction of weld metal.
Weld Decay
Weld Decay
Characteristics
Weld decay may occurs in unstabilized austenitic stainless
steels with carbon content above 0.1%
Also
Chromium carbide precipitation takes place at the critical range of 450oC-850oC (sensitising temperature )
At
this temperature range carbon is absorbed by the chromium, which causes a local reduction in chromium content by promoting chromium carbides.
Loss of chromium content results in lowering the materials resistance to corrosion attack allowing rusting to occur
Fatigue Cracks
Fatigue Testing
Fatigue Cracks
Fatigue cracks occur under cyclic stress conditions
Fracture normally occurs at a change in section, notch and weld defects i.e stress concentration area All welded materials are susceptible to fatigue cracking Fatigue cracking starts at a specific point referred to as a initiation point The fracture surface is smooth in appearance sometimes displaying beach markings
Fatigue Cracks
Secondary mode of failure ductile fracture rough fibrous appearance Fatigue fracture surface smooth in appearance
Fatigue Cracks
2) 45 Ductile fracture
2) 45 Ductile fracture.
2 3
3) Ductile fracture
3
3
a
2
b
2
c
3
Initiation point.
2 1
2
1) Characteristic Fatigue radius & fracture. Initiation point is on the edge of the shaft.
Copyright 2003 TWI Ltd
1
2 2 2
Characteristic chevrons.
Questions
QU 1. Briefly discuss the four essential factors for hydrogen cracking to occur QU 2. State four precautions to reduce the chance of hydrogen cracking QU 3. In which type of steel is weld decay is experienced and state how it can be prevented