Surface cracking mechanism of continuously cast low carbon low alloy steel slabs

Abstract

The present state of understanding of surface cracking in low C low alloy steel slabs in the continuous casting (CC) and direct rolling (DR) processes is outlined. Hot cracking of the CC slab surface can be explained in terms of carbide and/or nitride precipitation behaviour. In addition to γ grain boundary precipitation, the matrix strengthening owing to dynamic precipitation and the existence of softer layers along the boundaries such as grain boundary allotriomorphs of ferrite or precipitate free zones play animportant role in intergranular ductile fracture. The origin of hot cracking during the DR process lies also in the precipitation of carbides and/or nitrides, and is not related to the severe embrittlement caused by a similar mechanism with dynamic precipitation of sulphides, which is observed usually in the high strain rate deformation after reheating at higher temperatures. Furthermore, a well known effect of C on hot cracking susceptibility in both CC and DR processes, attaining a maximum in the range 0·10–0·15 wt.–%C, is found to arise mainly from γ grain growth during solidification in the mould. Some methods to prevent surface cracking are also discussed.

MST/1226     

Relationship between microstructure and hydrogen induced cracking behavior in a low alloy pipeline steel

Hydrogen induced cracking(HIC) behaviors of a high strength pipeline steel with three different microstructures, granular bainite lath bainite(GB + LB), granular bainite acicular ferrite(GB + AF), and quasi-polygonal ferrite(QF), were studied by using corrosion experiment based on standard NACE TM0284. The HIC experiment was conducted in hydrogen sulfide(H_2S)-saturated solution. The experimental results show that the steel with GB + AF and QF microstructure present excellent corrosion resistance to HIC, whereas the phases of bainite lath and martensite/austenite in LB + GB microstructure are responsible for poor corrosion resistance. Compared with ferrite phase, the bainite microstructure exhibits higher strength and crack susceptibility of HIC. The AF + GB microstructure is believed to have the best combination of mechanical properties and resistance to HIC among the designed steels.     

A case study of environmental assisted cracking in a low alloy steel under simulated environment of pressurized water reactor

The electromechanical behavior of a pressure vessel grade steel A516 has been investigated using potentiodynamic polarization curves and slow strain rate test (SSRT) in simulated environment of pressurized water reactor. The anodic polarization behavior shows that the steel remains active in the solution till localized attack (pitting) starts. The cracks initiated at the surface propagate in a trans-granular mode. These cracks are initiated at the inclusion (MnS) sites and at the interfaces between local anode (ferrite) and local cathode (pearlite). It seems that the ultimate fracture occurs when the propagating surface cracks join the subsurface hydrogen induced cracks. The addition of oxygen in the testing chamber to supersaturation levels shifts the corrosion potential to anodic side and significantly lowers the strength and ductility. Compared to the room temperature properties, the UTS and tensile elongation in various simulated conditions are reduced by 10–25% and 25–75%, respectively.     

Process and mechanism of fracture in a pipeline material in chloride and sulphide environments

The kinetics and mechanism of subcritical cracking in a hot rolled microalloyed steel have been investigated in presence of 3.5% NaCl and H₂S impregnated acetic acid solutions. The J-integral parameter is used to determine the threshold J_Iscc and terminal J_If values analogus to K_Iscc and K_If. H₂S environment appears to be more aggressive than the NaCl medium where considerable amount of energy dissipation occurs during the cracking process. Joining of fine cracks, resulting from blistering of pits/voids is responsible for subcritical cracking under H₂S medium, while the process of environmental cracking under NaCl medium occurs mainly by the growth of pits to link up in the direction of stressing.     

Stress corrosion cracking of a titanium alloy in chloride-containing liquid environments

Ti-8 Al-1 Mo-1 V was tested in solvents containing chloride, bromide or iodide ions. It was found that the region II plateau velocity could be correlated with the inverse of the viscosity for several solvents. It seems most likely that this reflects the influence of the viscosity on the diffusion coefficient of some species involved in a mass transfer process which limits the crack growth. Associated tests were performed in which the potential, pH and concentration of Cl^? were varied, in some of the same solutions as used for the viscosity tests. It is concluded from these tests that theabsence of cathodic protection against cracking in solutions of low pH arises from an ohmic effect which isolates the crack tip from external potential control.     

Sulfide stress cracking resistance of API-X100 high strength low alloy steel

Sulfide Stress Cracking (SSC) of API-X100 high strength low alloy steel was evaluated in NACE solution “A” at room temperature. The corrosion rate, utilizing electrochemical polarization techniques, in the solution was 97 mpy. Proof ring testing, per NACE TM-0177, generated an SSC threshold stress value of 46% of yield strength. SSC susceptibility was caused by the high corrosion rate which formed corrosion pits that acted as crack initiation sites on the metal surface and provided more hydrogen to migrate into the steel. In addition, the X100 inhomogeneous microstructure provided a high density of hydrogen traps which promoted hydrogen embrittlement.     

Small fatigue crack initiation and growth behavior of high-strength low alloy steel in various environments

Fatigue tests under rotating bending and reversed torsion were carried out in air, distilled water and 3% saltwater, using smooth specimens of high-strength low alloy steel (Cr-Mo steel). The initiation and growth behavior of small fatigue cracks in each environment were evaluated based on detailed observations, and the effects of corrosive environment were also discussed. The fatigue strength decreased with increasing aggressiveness of test environment. The decreases in corrosive environment were due to earlier fatigue crack initiation. From the observed locations at which small fatigue cracks began, it was considered that the crack initiation was primarily governed by hydrogen embrittlement in distilled water and also affected by corrosive dissolution in 3% saltwater. The validity of the application of linear fracture mechanics for small fatigue cracks was established. The growth rates of small fatigue cracks were higher than for large through cracks, and not accelerated by the corrosive environment. Moreover, fatigue life in the corrosive environment was estimated by using the crack growth characteristics in air.     

Some effects of yield strength on the stress corrosion cracking behaviour of low alloy steels in aqueous environments at ambient temperatures

The present paper describes the results of a literature review on the effects of material yield strength on the threshold stress corrosion cracking (SCC) condition known as K_1SCC, of a series of low alloy steels in various aqueous environments at ambient temperatures. It has been shown that an increase in yield strength reduced the value of K_1SCC and that this effect was more significant over the lower yield strength range 600–1200 MPa; at higher yield strength levels the effects of yield strength were much less evident. A review of the results indicated that SCC transgranular fracture was seen at yield strength values below around 1200 MPa while above this value the fracture path was exclusively intergranular in nature. Note that the transition point between transgranular and intergranular SCC coincided with the point at which yield strength effects were much reduced. A series of models reported in the literature, which attempted to explain the effects of yield strength on K_1SCC in terms of a fracture mechanics framework were examined with the introduction of a critical distance concept. Finally, the predictions of a grain size effect on the point at which the fracture path changed from transgranular to intergranular were taken from one model and used to show that the large data scatter in the region where transgranular fracture was operative, viz., the lower yield strength region of 600–1200 MPa, could be the result of grain size variations between the different reported studies.     

Resistance of a zirconium alloy to corrosion cracking under stresses

We describe a procedure for ultrahigh-frequency thermal treatment of é110 alloy and present the results of its textural and structural analysis. We also study the influence of the modes of ultrahigh-frequency treatment of the zirconium alloy on its long-term strength at a temperature of 653 K in corrosive media. The results of fractographic investigation of the fracture surfaces allow us to determine the mechanisms of initiation and propagation of stress-corrosion cracks in shells made of the alloy under consideration in the intact state and after ultrahigh-frequency treatment. Translated from Problemy Prochnosti, No. 2, pp. 122 – 130, March – April, 1998.     

Cyclic response of a 1Cr-Mo-V low alloy steel

The high strain fatigue behaviour of a bainitic (‘Non-heat-treated’) and a tempered bainitic (‘Heat-treated’) alloy steel has been studied at room temperature and 565°C. Cyclic softening is observed in all cases and is associated with dislocation redistribution, alignment and channelling of precipitates, and their growth at 565°C. Comparison of the appropriate monotonic and cyclic stress/strain curves enables a tentative estimate of the relative significance of these mechanisms to be made.     

Kinetics of niobium carbide precipitation in a low carbon austenitic steel

An examination has been made of the kinetics of niobium carbide precipitation in a 18-10-1 austenitic stainless steel in the temperature range 650 to 750° C. Electrical resistance-time plots, thin film electron microscopy and hardness measurements have been employed to follow the ageing sequence. In these alloys the carbides precipitate on undissociated dislocations and in association with stacking faults; these processes are diffusion controlled and have an activation energy of ～318 kJ mol^?1. Prior to the reaction beginning a clear incubation period existed, e.g. ～30 h at 650° C and 20 min at 750° C. During the first 10% transformation the carbide nucleation rate increases and the associated faults nucleate and grow rapidly. The carbide nucleation rate appears to peak around this level and then falls away gradually to zero around 70% transformation. At this latter stage fault growth ceases, and transformation continues by a carbide growth process. The age-hardening peak occurs much beyond the end of the reaction by which time precipitate coarsening is in evidence. The precise effect mechanical deformation has upon stacking fault formation depends to a major extent on the niobium supersaturation in the quenched alloy.     

Phosphorus grain boundary segregation under different applied tensile stress levels in a Cr-Mo low alloy steel

Phosphorus grain boundary segregation during 100 MPa stress ageing at 520 °C in a 2.25Cr1Mo steel is observed using Auger electron spectroscopy. In the segregation kinetics there is only a depletion trough of phosphorus below its thermal equilibrium level. Combined with preceding experimental results for other stress levels, the evolution of segregation kinetics with stress is addressed. The basic features of the segregation kinetics include two segregation peaks and one depletion trough. The first peak, the second peak and the trough may be related to the vacancy-solute complex effect, the boundary diffusion effect and the creep deformation inhomogeneity effect, respectively.     

The abrasive corrosion behavior of plasma-nitrided alloy steels in chloride environments

Both corrosion and abrasive corrosion behavior of plama-nitrided type 304 and 410 stainless steels and 4140 low alloy steel were investigated in 3% NaCl solution (pH = 6.8) by electrochemical corrosion measurements. Surface morphology and alloying elements after corrosion and abrasion corrosion tests were examined by scanning electron microscopy and energy dispersive analysis of X-rays. The results indicated that the plasma-nitrided SAE 4140 steel containing -(Fe,Cr)_{2 – 3}N and -(Fe,Cr)₄N surface nitrides which produce a thick and dense protective layer exhibited a significant decrease of corrosion currents by inhibition of the anodic dissolution of iron, whereas the plasma-nitrided type 304 and 410 stainless steels containing the segregation of chromium nitride CrN exhibited a extensive pitting corrosion by acceleration of the anodic dissolution of iron. It is concluded that the susceptibility to pitting is consistent with the degree of chromium segregation, and decreases as follows: 304 stainless steel > 410 stainless steel > 4140 steel. Also, the results of abrasive corrosion testing for the plasma-nitrided alloys are strongly related to the subtleties of the nitrided microstructures resulting in a pitting and spalling type of abrasive corrosion of type 304 and 410 stainless steels, and excellent abrasive corrosion resistance for SAE 4140 steel.     

Stress corrosion cracking of AISI 321 stainless steel in acidic chloride solution

The stress corrosion cracking (SCC) of AISI 321 stainless steel in acidic chloride solution was studied by slow strain rate (SSR) technique and fracture mechanics method. The fractured surface was characterized by cleavage fracture. In order to clarify the SCC mechanism, the effects of inhibitor KI on SCC behaviour were also included in this paper. A study showed that the inhibition effects of KI on SCC were mainly attributed to the anodic reaction of the corrosion process. The results of strain distribution in front of the crack tip of the fatigue pre-cracked plate specimens in air, in the blank solution (acidic chloride solution without inhibitor KI) and in the solution added with KI measured by speckle interferometry (SPI) support the unified mechanism of SCC and corrosion fatigue cracking (CFC).