Susumu Tsukamoto

Suppressing type IV failure via modification of heat affected zone microstructures using high boron content in 9Cr heat resistant steel welded joints

Abstract Creep rupture strength at 923 K and microstructural evolution of welded joints have been investigated for high boron–low nitrogen–9Cr heat resistant steels developed at the National Institute for Materials Science (Japan). Welded joints were prepared from plates containing 47–180 ppm boron using gas tungsten arc welding and Inconel type filler metal, and showed superior creep properties to those of welded joints of conventional high chromium steels such as P92 and P122. No type IV failure was observed in the boron steel welded joints. A large grained microstructure was observed in the heat affected zone heated to Ac 3 (Ac 3 HAZ) during welding, whereas the grains are refined at the same location in conventional steel welded joints. The simulated Ac 3 HAZ structures of the boron steels have a creep life almost equal to that of the base metal. Large grained HAZ microstructures and stabilisation of M23C6 precipitates are probable reasons for suppression of type IV failure and improved creep resistance of the boron steel welded joints.

High speed imaging technique Part 2 – High speed imaging of power beam welding phenomena

Abstract The high speed imaging technique is an attractive tool to elucidate welding phenomena. In this paper, applications of this technique are reviewed in order to understand the power beam welding phenomenon. Monochromatic imaging technique, which can take pictures of plasma with any specific spectrum wavelength, is used to analyse state of the laser induced plasma and distribution of some species, such as ions and atoms of gases and metals. During electron beam welding, intermittent melting process and spiking phenomenon can be clearly recorded by a pinhole X‐ray streak camera. The in situ X‐ray transmission imaging system is useful to observe the dynamic keyhole and fluid flow behaviour, which cannot be directly observed by any other method. High speed observations of the solidification front clearly show the solidification cracking process. In the present paper, methods to analyse the solidification cracking using high speed imaging are also described.

Influence of welding parameters on distribution of wire feeding elements in CO2 laser GMA hybrid welding

Abstract The effect of welding parameters on the distribution of wire feeding elements has been investigated during CO2 laser and pulsed gas metal arc hybrid welding process. The molten metal flow on the pool surface and inside of the samples was observed by a high speed video camera and an in situ X-ray transmission imaging system respectively. The results indicate that the fluid flow towards the inside of keyhole, namely inward flow, improves the homogeneity of weld metal. The distribution of alloying elements is more homogeneous in leading laser compared with leading arc, since both of the drag force of the plasma jet and momentum of droplet promote the inward flow in leading laser. Almost homogeneous distribution of alloying elements can be attained if the oxygen content in the shielding gas is more than 2%, since the Marangoni flow direction changes from outward to inward with increasing the oxygen content.

Ferrite to austenite reverse transformation process in B containing 9%Cr heat resistant steel HAZ

Abstract Creep properties of the high Cr heat resistant steel welded joint can be improved by adding B due to prevention of the grain refinement in heat affected zone (HAZ). In the present study, phase transformation behaviour of the B steel HAZ has been investigated to understand suppression mechanism of the grain refinement. During reverse transformation, fine austenite was formed through diffusional transformation at the prior austenite grain boundary in the first stage, and then coarse austenite was formed at the same location of the original austenite. The volume fraction of the fine austenite increased with increasing perk temperature of the weld thermal cycle. This phenomenon can be explained if the coarse austenite contains high density of dislocations. Clear surface relief was observed during the reverse transformation by a confocal laser microscope. These results indicate that martensitic or displacive reverse transformation takes place during welding and it prevents the grain refinement in HAZ.

Mechanisms for boron effect on microstructure and creep strength of ferritic power plant steels

Abstract Boundary and sub-boundary hardening are shown to be the most important strengthening mechanism in creep of 9%Cr steel. Soluble boron reduces the coarsening rate of M23C6 carbides near prior austenite grain boundaries during creep, enhancing the boundary and sub-boundary hardening for long times at 650°C. The enhancement of boundary and sub-boundary hardening retards the onset of acceleration creep, which decreases the minimum creep rate and improves the creep life. Excess addition of boron and nitrogen promotes the formation of boron nitrides during normalising heat treatment, which significantly reduces soluble boron and soluble nitrogen concentrations. The boundary and sub-boundary hardening are significantly reduced in the fine grained region of the heat affected zone (HAZ) of Gr.92 welded joints, promoting type IV fracture. Soluble boron produces substantially the same microstructure between base metal and HAZ in 9Cr steel welded joints, resulting in no type IV fracture at 650°C.

Formation Mechanism of Type IV Failure in High Cr Ferritic Heat-Resistant Steel-Welded Joint

The mechanism of type IV failure has been investigated by using a conventional 9Cr ferritic heat-resistant steel Gr.92. In order to clarify the main cause of type IV failure, different heat treatments were performed on the base metal in order to change the prior austenite grain (PAG) size and precipitate distribution after applying the heat-affected zone (HAZ) simulated thermal cycle at the peak temperature of around Ac3 (Ac3 HAZ thermal cycle) and postweld heat treatment (PWHT). The microstructural evolution during the Ac3 HAZ thermal cycle and PWHT was investigated by means of scanning electron microscope (SEM), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA), and transmission electron microscope (TEM). It was found that M23C6 carbides were scarcely precipitated at the newly formed fine PAG, block, and lath boundaries in Ac3 HAZ-simulated Gr.92, because the carbide forming elements such as Cr and C were segregated at the former PAG and block boundaries of the base metal. On the other hand, if all the boundaries were covered by sufficient M23C6 carbides by homogenization of the alloying elements prior to applying the HAZ thermal cycle, the creep strength was much improved even if the fine PAG was formed. From these results, it is concluded that fine-grained microstructure cannot account for the occurrence of type IV failure, and it only has a small effect during long-term creep. The most important factor is the precipitate formation behavior at various boundaries. Without sufficient boundary strengthening by precipitates, the microstructure of Ac3 HAZ undergoes severe changes even during PWHT and causes premature failure during creep.

Role of Boundary Strengthening on Prevention of Type IV Failure in High Cr Ferritic Heat-Resistant Steels

Microstructure evolution of newly developed 9Cr-3W-3Co-V, Nb steel with boron addition (B steel) has been analyzed during HAZ thermal cycle at the peak temperature of around Ac3 (Ac3 HAZ) and post-weld heat treatment (PWHT) to elucidate the prevention mechanism of type IV failure by boron addition. It was found that enhancement of the boundary strengthening by precipitates is the main reason for prevention of type IV failure by boron addition. In B steel HAZ, original austenite is reconstituted through martensitic α to γ reverse transformation during the heating and original martensite is reconstituted through martensitic transformation during cooling of the Ac3 HAZ thermal cycle. This process allows M23C6 carbides to precipitate at the prior austenite grain (PAG) and block boundaries during PWHT even if the chemical segregation of carbide forming elements exists. The effect of boundary strengthening on the creep property has also been investigated. Microstructure evolution during creep was compared among Gr.92 with different Ac3 HAZ microstructures prepared by three kinds of heat treatments and B steel. The results revealed that both the boundary length and kernel average misorientation value decreased in all samples during creep. However, this process occurred very rapidly in Ac3 HAZ simulated Gr.92, whereas it was significantly retarded in the other samples with sufficient boundary strengthening by precipitates. This result confirms that the precipitates formed at PAG and block boundaries play the most important role to stabilize the microstructure of Ac3 HAZ simulated samples during creep and prolong the creep life.

Keyhole behavior in high-power laser welding

Dynamic keyhole behavior has been observed to elucidate the formation and suppression mechanism of the porosity in 20 kW CO2 laser welding with the depth of 20 mm. The results indicate that the bubble is formed by capillary instability of the cylindrical keyhole. The tip of the keyhole is broken up by instability during rapid decrease in the depth, so called spiking phenomenon. Spontaneous fluctuation in the keyhole depth and spontaneous keyhole perturbation during welding promotes the bubble formation. Pulse modulation of the laser power is effective in stabilizing the keyhole and hereby suppressing the porosity if the frequency coincides with the eigenfrequency of the molten pool oscillation. The suppression effect is enhanced if the waveform is controlled appropriately.

Precipitation Behavior in the Heat-Affected Zone of Boron-Added 9Cr-3W-3Co Steel During Post-Weld Heat Treatment and Creep Deformation

In the previous paper, we demonstrated that the addition of boron was effective in preventing type IV failure due to suppression of grain refinement in the heat-affected zone at the peak temperature of around AC3 (AC3 HAZ). However, some fine prior austenite grains (PAGs) still remained around the coarse PAG boundaries, and these fine PAGs may affect the creep property of the welded joint. In the present study, the effect of these fine PAGs on the creep property of the boron-added 9Cr-3Co-3W steel (B steel) Ac3 HAZ is investigated. Different heat treatments are carried out on B steel base metal to form different Ac3 HAZ-simulated microstructures of coarse PAG with and without fine PAGs. Ac3 HAZ microstructure shows that a lot of M23C6 carbides are formed at the block boundary in the interior of coarse PAG. On the other hand, few M23C6 carbides are formed at the fine PAG boundaries, but a number of μ phases (W6Fe7 type) cover the boundary. The formation of μ phase retards the recovery of dislocation at the fine PAG boundary and contributes to stabilizing the microstructure in the primary and transient creep regions. The μ phase transforms to the Laves phase during creep. As the growth rate of Laves phase is higher than that of M23C6 carbides during creep, the creep strength of fine PAG boundary, which is strengthened only by Laves phase, becomes a little bit lower than the other boundaries strengthened by M23C6 carbides after long-term creep. The mismatch of creep strength between the fine PAG boundary and the matrix should be taken into account to attain an excellent long-term creep property of the B steel welded joint.

Influence of oxygen on weld geometry in fibre laser and fibre laser-GMA hybrid welding

Abstract The effect of oxygen on weld geometry during keyhole mode welding has been investigated by adding a small amount of oxygen to the shielding gas during fibre laser and fibre laser–gas metal arc hybrid welding. The results indicate that the penetration depth increases and the weld width decreases with increasing oxygen concentration. This effect is attributed to the formation of a deeper keyhole when oxygen is present. The addition of sulphur up to 1500 ppm in the molten pool has no significant effect on the penetration depth. This behaviour indicates that the Marangoni convection and surface tension are not the main reasons for the deeper weld penetration when oxygen is added to the shielding gas. The increase in the penetration depth owing to oxygen addition is consistent with the formation of CO by reaction between dissolved carbon and oxygen. Rapid generation of CO in the keyhole expands the keyhole and results in deeper weld penetration.