Abstract
High strength low alloy steels (HSLA) are currently used in large quantities for constructions in the North Sea and elsewhere. For success operation under such rigorous conditions, it is essential to ensure adequate weld metal toughness at temperatures of -10ºC or even lower is used. To comply with this high requirement, the heat input control is needed to avoid forming martensitic microstructure in high strength low alloy steels (HAZ) near to the fusion line during weldment cooling. This work carried out CTOD tests to ensure that, under welding conditions, the fatigue crack, toughness, hardness, and resilience in the HAZ are acceptable with the standards. Crack size in accordance with number cycles will be represented as the crack growth rate da/dN vs. stress intensity factor amplitude.
TopIntroduction
One problem that design of these steels of high resilience have shown is their low toughness in the HAZ Schuller, R., et al. (2015), when they are welded with a high heat input (HI). In this work we have studied nine specimens that have been welded under three different process:
SAW (submerged arc welding), SMAW(shield metal arc welding), GSFC(gas shield flux core arc welding) controlling the welding parameters and checking in the HAZ of such specimens, critical tensions (σcr)at the ends of the cracks, the critical cracks lengths (acr) and stress intensity factors (KIC).It is intended to check that the parameters that indicate the values of fracture mechanics:
, da/dN=A ΔKm, etc.,) in the HAZ, after heat cycle to which the steel has undergone, under a process with a maximum “heat input” of 2.327kJ /mm (see Table 5 below), are still valid, with the welding parameters applied. It is checked a correlation between the theoretical values and those obtained experimentally. Experimental work, here conducted, we intend to ensure that these parameters regulating the mechanism of fracture, remain valid under the rules compatible with the design of the base material and that as far as possible be established a correlation between welding parameters and those obtained in fracture tests, so that if the results of the tests are not satisfactory, we could apply the appropriate solution in welding for the parameters governing fracture tests were acceptable. We will check as the HAZ region behave as ferritic-Pearlitic steels complying with the equation:
The tests of CTOD, resilience and hardness were carried out on the panel numbered as CTOD HAZ. Said panel, 60 mm thick, 980 mm weld length and 750 mm wide, was tested under “As welded” conditions (without post-weld heat treatment); and it was used to test the HAZ. The welding was done in K and V, in three different process Shield Metal Arc Welding (SMAW) in the panel 3 AW where from taken the samples 01-1(1), 01-2(2) and 01-3(3). The second process was Gas Shield Flux Core (GSFC)in the panel 7 AW where from were taken the specimens 02-01(4), 02-02(5) and 02-03(6) and the third in the process Submerged Arc Welding (SAW) the panel 01 AW where from the following panel were taken 01-10(7), 01-11(8) and 01-12(9). The base metal was BS 7191(1979) 450 EMZ TYPE 2.
The thermal cycle to which these test pieces were subjected, for the effects of the transformation products that are produced in the cooling inside the HAZ with a contributed maximum energy of 2.327 KJ / mm, a peak fusion temperature is obtained 1,527 °C, a cooling time Δt8 /5, practically constant for the entire HAZ, of 10 seconds, as shown by Rosenthal's theoretical cycle for this thick plate process, González-Palma et al (2006). The physical characteristics of the 450 EMZ micro alloy steel are given in the casting certificate.
The life of the components of a structure containing premature cracks, can be governed by the degree of subcritical crack propagation. To this end, the many destructive and non-destructive testing can collaborate to establish any cracks before commissioning. Thus, knowledge of crack propagation through the concepts of fracture mechanics calculation to determine the fatigue of the structure is necessary. However, after the thermal cycle in a process, material suffers, the mechanism of the microstructure of the base metal can change, being able to modify the parameters such and to the extent that if not properly controlled, could lead to catastrophic failure of the structure.
Key Terms in this Chapter
Stress Intensity Factor Amplitude: It is the fluctuation of the stress intensity factor ? K , between a K max and K min , which can be called as stress intensity factor range. At the threshold it represented as ? K th .
Total Crack: It is represented as a = d a + a m .
GSFC: Gaz Shield Flux Core It is semiautomatic welding where the weld it is protected with gaz
Critical Crack: It is represented as a cr and is the crack is produced in the first part of the testing is made by fatigue the testing stop when 0.45< a m / W <0.55.
ASTM Standard E 112: Stablish a correspondence with the grain size classifying them by a number that this standard represents between 0 (508 µm) and 14 (2.8µm) average diameter. Considering that base metal (non-affected zone) has a size of 12 ASTM (5µ) average diameter approximately.
SAW: Submerged Arc Welding. Automatic welding process.
HEYN: It is a method to measure the grain size in the heat affected zone of steel after welding, observed by microscope, grain size in accordance with the ASTM (112-63) standard.
FGHAZ: Fine grain, it is region within the heat affected zone of the metal that is being welded, where the temperature reached is between 900°C and 1100°C
Stress Intensity Factor: It is a dimensional constant which gives the magnitude of the elastic stress field. It is represented by K . Dimensional analysis shows that K must be linearly related to stress and directly related to the square root of a characteristic length is a dimensionless parameter that depends on the geometric of the specimen and crack.
HI: Heat input, heat energy, it depends of electrical Intensity and voltage of the welding arc and the speed of the torch. in the HSLA steel is limited to 2.6 kJ/mm. (V×I/v).
Slow Stable Crack Extension: It is represented as d a is difference between a - a m , being a m = a 0 + a cr , where a cr is the critical crack.
Initial Crack: It is the mechanical notch made by saw represented as a 0 . The length (M)= a 0 equal to the width of the specimen (W) see Figure11 .
SMAW: Shield Metal Arc Welding. It is manual welding using electrodes.
AC3: Region of peak temperature at 900°C.
SC: It is region within the heat affected zone of the metal that is being welded, where the temperature reached is lower than 720ºC.
Resilience: Is the ability of a material to absorb energy when it is deformed elastically, and release that energy upon unloading.
HSLA: High strength low alloy steel, they are microalloyed steels with high yield point. they have low content of carbon compensated with microalloys.
Charpy CVN: It is a testing to take measure of resilience, if this testing fails, the CTOD testing not need to be and the sample is rejected.
CTOD: Crack Tip Opening Displacement this testing it is to measure toughness in HAZ or weld metal, this testing require Charpy CVN has passed the testing.
Toughness: The elasticity and hardness of a metal object; its ability to absorb considerable energy and undergo considerable deformation before cracking.
Fracture Toughness: It is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design applications. Here it is measured by CTOD testing.
AC1: Region of peak temperature at 720°C.
IRGCHAZ: Intercritical grain coarse zone, it is region within the heat affected zone of the metal that is being welded, where the temperature reached is between 720°C and 900°C. Reheated by passes of welding.
AW: As welded conditions, metal without post-weld heat treatment.
Hardness: Hard Vickers 10 to determine hard points in HAZ or weld metal.
FL: Fusion line. It is the line between the welding and heat affected zone.
Stress Intensity Factor Threshold: It is the stress intensity factor at which the crack propagates.
GCHAZ: Grain coarsened heat affected zone, it is region within the heat affected zone of the metal that is being welded, where the temperature reached is between 1100ºC and 1450ºC. It is a fragile zone.