Thermal techniques - Distortion - corrective techniques

Residual Stress and Distortion

10 Residual Stress and Distortion .1 What causes distortion?

10.8 Distortion - corrective techniques

10.8.2 Thermal techniques

The basic principle behind thermal techniques is to create sufficiently high local stresses so that, on cooling, the component is pulled back into shape.

Figure 10.15 Localised heating to correct distortion.

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This is achieved by locally heating the material to a temperature where plastic deformation will occur as the hot, low yield strength material tries to expand against the surrounding cold, higher yield strength metal. On cooling to room temperature the heated area will attempt to shrink to a smaller size than before heating. The stresses generated thereby will pull the component into the required shape (see above).

Local heating is, therefore, a relatively simple but effective means of correcting welding distortion. Shrinkage level is determined by size, number, location and temperature of the heated zones. Thickness and plate size determines the area of the heated zone. Number and placement of heating zones are largely a question of experience. For new jobs, tests will often be needed to quantify the level of shrinkage.

Spot, line, or wedge-shaped heating techniques can all be used in thermal correction of distortion.

Spot heating

Figure 10.16 Spot heating for correcting buckling.

Spot heating is used to remove buckling, for example when a relatively thin sheet has been welded to a stiff frame. Distortion is corrected by spot heating on the convex side. If the buckling is regular, the spots can be arranged symmetrically, starting at the centre of the buckle and working outwards.

Line heating

Figure 10.17 Line heating to correct angular distortion in a fillet weld.

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Heating in straight lines is often used to correct angular distortion, for example, in fillet welds. The component is heated along the line of the welded joint but on the opposite side to the weld so the induced stresses will pull the flange flat.

Wedge-shaped heating

To correct distortion in larger complex fabrications it may be necessary to heat whole areas in addition to employing line heating. The pattern aims at shrinking one part of the fabrication to pull the material back into shape.

Figure 10.18 Use of wedge shaped heating to straighten plate.

Apart from spot heating of thin panels, a wedge-shaped heating zone should be used from base to apex and the temperature profile should be uniform through the plate thickness. For thicker section material, it may be necessary to use two torches, one on each side of the plate.

As a general guideline, to straighten a curved plate wedge dimensions should be:

 Length of wedge - two-thirds of the plate width.

 Width of wedge (base) - one sixth of its length (base to apex).

The degree of straightening will typically be 5mm in a 3m length of plate.

Wedge-shaped heating can be used to correct distortion in a variety of situations, (see below):

 Standard rolled section, which needs correction in two planes a).

 Buckle at edge of plate as an alternative to rolling b).

 Box section fabrication, which is distorted out of plane c).

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Figure 10.19 Wedge shaped heating to correct distortion.

General precautions

The dangers of using thermal straightening techniques are the risk of over-shrinking too large an area or causing metallurgical changes by heating to too high a temperature. As a general rule, when correcting distortion in steels the temperature of the area should be restricted to approximately to 600-650°C - dull red heat.

If the heating is interrupted, or the heat lost, the operator must allow the metal to cool and then begin again.

Best practice for distortion correction by thermal heating

The following should be adopted when using thermal techniques to remove distortion:

 Use spot heating to remove buckling in thin sheet structures.

 Other than in spot heating of thin panels, use a wedge-shaped heating technique.

 Use line heating to correct angular distortion in plate.

 Restrict the area of heating to avoid over-shrinking the component.

 Limit the temperature to 600-650°C (dull red heat) in steels to prevent metallurgical damage.

 In wedge heating, heat from the base to the apex of the wedge, penetrate evenly through the plate thickness and maintain an even temperature.

a) Standard rolled steel

section b) Buckled edge of plate c) Box fabrication

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10‐1

Section 10

Residual Stress and Distortion

Residual stresses are undesirable because

 They lead to distortions.

 They affect dimensional stability of the welded assembly.

 They enhance the risk of brittle fracture.

 They can facilitate certain types of corrosion.

Factors affecting residual stresses

 Parent material properties.

 Amount of restrain.

 Joint design.

 Fit-up.

 Welding sequence.

Residual Stress

Parent material properties

 Thermal expansion coefficient - the greater the value, the greater the residual stress.

 Yield strength - the greater the value, the greater the residual stress.

 Young’s modulus - the greater the value (increase in stiffness), the greater the residual stress.

 Thermal conductivity - the higher the value, the lower the residual stress.

Factors Affecting Residual Stress

Joint design

 Weld metal volume.

 Type of joint - butt vs. fillet, single vs. double side.

Amount of restrain

 Thickness - as thickness increase, so do the stresses.

 High level of restrain lead to high stresses.

 The lack of pre heat will increase stresses.

Fit-up

 Misalignment may reduce stresses in some cases.

 Root gap - increase in root gap increases shrinkage.

Factors Affecting Residual Stress

Welding sequence

 Number of passes - every pass adds to the total contraction.

 Heat input - the higher the heat input, the greater the shrinkage.

 Travel speed - the faster the welding speed, the less the stress.

 Build-up sequence.

Factors Affecting Residual Stress

Residual stresses

 Are a result of local plastic deformation.

 Are a result of non uniform heating and cooling ie welding.

 Are a result of non uniform heating, cooling, expansion and contraction.

 This is because the expansion and contraction can be obstructed by colder surrounding materials and also the mechanical properties of the material being welded.

Factors Affecting Residual Stress

10‐2

Heating and cooling leads to expansions and contractions.

The material as shown can expand and contract freely without hindrance.

A welded joint does not react in this way!

Nature of Residual Stress

If expansion is hindered, compressive stresses occur.

If on cooling shrinkage is obstructed, tensile stresses occur.

The overall result, Residual Stresses.

Residual Stress

Origins of residual stress in welded joints Residual Stress

Hot weld

Cold weld unfused

Cold weld fused

Residual stresses

 Temperatures higher than 600°C, depending on the restraint, plastic deformation occurs (distortion).

 Temperatures lower than 600°C, depending on restraint, residual stresses occur because temperature not high enough to yield the material sufficiently.

Factors Affecting Residual Stress

Longitudinal residual stress after welding

The longer the weld, the higher the tensile stress!

Types of Residual Stress

Tension

Compression

Maximum stress = YS at room temperature

Residual stress after welding

The higher the heat input the wider the tensile zone!

Types of Residual Stress

Tension Compression

YS at room temperature

10‐3

Reducing residual stresses

 The most effective way to reduce residual stresses is to post weld heat treat uniformly.

 The most effective method is to PWHT the whole member but this is not always possible.

 A controlled local, uniform PWHT usually reduces stresses by 75%.

Residual Stress

Post weld heat treatment

 Controlled ramp up to soak temperature so that complex items are heated uniformly and distortion does not take place.

 Held at soak temperature for approximately one hour for every 25mm of thickness.

 Controlled reduction of temperature.

Residual Stress

Local heat treatment using electric heating blankets

Advantages

 Ability to vary heat.

 Ability to continuously maintain heat.

Disadvantages

 Elements may burn out or arcing during heating.

Heat Treatment Methods

HF local heat treatment

Advantages:

 High heating rates.

 Ability to heat a narrow band.

Disadvantages

 High equipment cost.

 Large equipment, less portable.

Heat Treatment Methods

TEMP

Factors affecting distortion

 Parent material properties.

 Amount of restrain.

 Joint design.

 Fit-up.

 Welding sequence.

Distortion

10‐4

Parent material properties

 Thermal expansion coefficient - the greater the value, the greater the residual stress.

 Yield strength - the greater the value, the greater the residual stress.

 Thermal conductivity - the higher the value, the lower the residual stress.

Factors Affecting Distortion

Welding sequence

 Number of passes - every pass adds to the total contraction.

 Travel speed - the faster the welding speed, the less the stress.

 Build-up sequence.

Factors Affecting Distortion

Angular distortion

Types of Distortion

Distortion prevention by design Consider eliminating the welding!!

a)By forming the plate.

b)By use of rolled or extruded sections.

Distortion Prevention

Distortion prevention by design

 Consider weld Placement.

 Reduce weld metal volume and/or number of runs.

Distortion Prevention

Distortion prevention by design

 Use of balanced welding.

Distortion Prevention

10‐5

Distortion prevention by fabrication techniques

Control welding techniques by a) Back-step welding.

b) Skip welding.

Distortion Prevention

You are currently employed as a Senior Welding Inspector on a fabricated steel structure.

The structure has many different joint configurations with a thickness range from 12.5mm up to 50mm.

All welding to be completed by either the SAW or MMA welding processes.

One of your main tasks is to ensure both stress and distortion is kept to a minimum.

Residual Stress and Distortion

Residual stresses would play a major part in which of the following

a. HICC and brittle fracture

b. Lamellar tearing and solidification cracking c. Fatigue and ductile failure

d. Chevron cracking and hot cracking

Question 1

Which of the following conditions would cause the greatest amount of distortion on this type of fabricated structure?

a. A highly restrained joint during welding b. A joint, which is free to move during welding c. A joint, which would be subjected to the

lowest heat input d. 2 options are correct

Question 2

Which combination of factors will increase the level of distortion?

a. High Rm, high thermal conductivity and low coefficient of expansion

b. Low Re, low thermal conductivity and high coefficient of expansion

c. High yield, high UTS and low coefficient of expansion

d. Low percentage Z, High percentage of Sulphur and Phosphorous

Question 3

The fabrication contains materials of varying Re values, generally which of the following would you expect to distort the most without control methods in place?

a. Welded joints made from the highest Re value materials

b. Welded joints made from the lowest Re value materials

c. Welded joints that contain the highest residual stress

d. 2 options are correct

Question 4

10‐6

Part of the fabrication contains a joint made from C/Mn steel welded to a 316L steel. Which of the following best applies when considering distortion?

a. The C/Mn steel side of the joint will distort the most due to high thermal expansion

b. The C/Mn steel side of the joint will distort the most due to low thermal conductivity

c. The 316L side of the joint will distort the most due to high thermal conductivity

d. The 316L side of the joint will distort the most due to low thermal conductivity

Question 5

Which of the following are factors affecting distortion?

a. Parent material properties b. Joint design/amount of restraint c. Heat input/welding sequence d. All options are correct

Question 6

The fabricator approaches you on the best way to reduce distortion. The joint configuration, welding process, material type can’t be changed. Which of the following could be applied to reduce distortion?

a. Increase restraint and minimize the amount of weld beads deposited, heavier weld beads b. Reduce restraint and minimize the amount of weld

beads deposited, heavier weld beads

c. Increase restraint and maximize the amount of weld beads deposited, lighter weld beads d. Reduce restraint and increase the amount of weld

beads deposited, heavier weld beads

Question 7

Which of the following thickness and joint configurations would you expect to produce the highest amount of distortion?

a. 25.5mm single V butt b. 50mm single U butt c. 50mm double U butt d. 25.5mm single J butt

Question 8

After welding it is a requirement to conduct a PWHT on certain welded joints. On this welded structure what is the main purpose of this heat treatment?

a. Normalising the material to increase the UTS value for the welded structure

b. For hydrogen release, especially if a E8016 electrodes had been used for the welding of the joint.

c. For stress relieving the welded joint d. To anneal and temper the weld metal

Question 9

One of your inspectors asks you what would a typical PWHT temperature be, when applied to this

fabrication. Which of the following would be the correct answer when taking into account the material thickness range stated on a C/Mn to C/Mn steel welded joint?

a. Approximately 50°C above the upper critical limit of the material stated

b. Between 600°C to 650°C

c. Approximately 100°C lower than the lower critical limit of the material stated

d. 2 options are correct

Question 10

Section 11

In document CSWIP 3.2 Course Material 2016 (Page 181-191)