Stress corrosion cracking of maraging steel weldments

Abstract: 4-(N,N-diethylamino)benzaldehyde thiosemicarbazone (DEABT) was studied for its corrosion inhibition property on the corrosion of aged 18 Ni 250 grade maraging steel in 0.67 M phosphoric acid at 30–50 °C by potentiodynamic polarization, EIS and weight loss techniques. Inhibition efficiency of DEABT was found to increase with the increase in DEABT concentration and decrease with the increase in temperature. The activation energy Ea and other thermodynamic parameters (Δ G ads 0 , Δ H ads 0 , Δ S ads 0 ) have been evaluated and discussed. The adsorption of DEABT on aged maraging steel surface obeys the Langmuir adsorption isotherm model and the inhibitor showed mixed type inhibition behavior.

Abstract: The corrosion inhibition of the aged 18 Ni 250 grade maraging steel in 0.5 M sulfuric acid by 3,4-dimethoxybenzaldehydethiosemicarbazone(DMBTSC) has been investigated by potentiodynamic polarization and electrochemical impedance spectroscopy(EIS) techniques. The inhibition efficiency increased with the increase in inhibitor concentration and decreased with the increase in temperature. Polarization curves indicated mixed type inhibition behavior affecting both cathodic and anodic corrosion currents. The thermodynamic parameters of corrosion and adsorption processes were evaluated. The adsorption of DMBTSC on the aged maraging steel surface was found to obey the Langmuir adsorption isotherm model, and the calculated Gibb’s free energy values confirm the spontaneous adsorption. The results obtained by the two techniques were in good agreement.

Abstract: Friction stir welding (FSW), a solid state welding process, is generally used for welding soft materials like aluminium and magnesium alloys. In this work, maraging steel, an ultra high strength steel, is welded using FSW process. Major problems associated with fusion welded maraging steel, i.e segregation of alloying elements and formation of reverted austenite pool, were suppressed by FSW. Slow strain rate stress corrosion characteristic of 18%Ni maraging steel (grade-250) and its weldments, by gas tungsten arc welding (GTAW) and FSW, has been investigated in air and 3.5% NaCl solution. It was observed that FSW joints demonstrated higher resistance to stress corrosion cracking (SCC) compared to both, base metal and gas tungsten arc weldments. Improved stress corrosion resistance of friction stir weldments was correlated to microstructural differences and residual stress. Fine grain structure, absence of segregation of alloying elements and high compressive residual stress in friction stir weld resulted in better SCC resistance compared to that of fusion weldment. It is brought out from the present study that FSW process can be used as an alternative to fusion welding process for better SCC resistance.

Abstract: This chapter reviews experimental studies of the hydrogen embrittlement of high strength steels. The focus has been on six types of steel, having distinctly different microstructures. The six steel types are the low alloy steels, high toughness secondary hardening steels such as AF1410, hot work die steels, martensitic secondary hardening stainless steels, maraging steels and precipitation strengthened martensitic stainless steels. The susceptibilities of these classes of steel to hydrogen embrittlement as measured by the effects of hydrogen on fracture initiation and subsequent crack growth are discussed when the steels are tested in hydrogen gas, when the hydrogen has been introduced by charging and when the hydrogen embrittlement is associated with stress corrosion cracking in distilled water or salt water.

Abstract: Slow strain rate testing of notched cylindrical specimens of 18Ni2400 maraging steel has been carried out in air with 30% relative humidity and synthetic seawater environments. Peak-aged condition has been chosen, considering the relevance to engineering applications. Studies have also been carried out with different notch geometries to understand the effect of stress concentration factor. It is concluded from the study that (i) degree of stress concentration at the notch influences the notched tensile strength (ii) mild hydrogen embrittlement seems to occur in air environment, (iii) synthetic seawater environment drastically brings down the notched tensile strength and time to fracture (iv) environmentally assisted cracking occurs in air tests in quasicleavage and microvoid coalescence modes and in seawater tests in intercrystalline mode.

Abstract: Most high strength steels are susceptible to stress-corrosion cracking (SCC) in aqueous environments if stressed in tension, but in the general case a stress raiser is required before SCC commences. By inserting a fatigue crack in the specimen before commencing the test, SCC can be caused to initiate immediately upon application of sufficient stress. The stress at the root of the precrack can be described quantitatively by the stress intensity parameter KI and a threshold value of this required to initiate SCC is defined as KIscc. A simple test to do this is described. The results are used to predict the effect of various small surface flaws on cracking of other specimens under various stresses, and the predictions are compared with experience with a martensitic steel of the AISI 4340 type.

Abstract: Maraging steels are a class of high-strength steels characterised by very low carbon contents and the use of substitutional elements to produce age-hardening in iron–nickel martensites. The term 'maraging' was coined from a combination of martensite and age-hardening.

Abstract: In the present work, slow displacement rate tensile tests were performed to find out the influence of ageing condition and hydrogen-charging on the notched tensile strength and fracture characteristics of T-250 maraging steel aged at various conditions. The influence of embrittling species in the environment on the notched tensile strength was accessed by comparing the measured properties in air, gaseous hydrogen and H2S-saturated solution. The hydrogen diffusivity, permeation flux and apparent solubility of various specimens determined by electrochemical permeation method, were correlated well with the microstructures and mechanical property. The results indicated that the peak-aged (H900) specimen was highly sensitive to hydrogen embrittlement even in gaseous hydrogen. In contrast, the microstructures of over-aged (H1100) specimen comprising of reverted austenite and incoherent precipitates could trap large amount of hydrogen atoms, resulting in decreased hydrogen permeability and hydrogen embrittlement susceptibility. The solution-annealed specimen had the highest diffusion coefficient and the lowest quantity of trapped hydrogen among the specimens, showing high susceptibility to sulfide stress corrosion cracking. In the presence of notches, hydrogen atoms were prone to segregate and trap at grain boundaries, resulting in the formation of intergranular fracture.

Abstract: The microstructure of oxides formed on 250 maraging steel in steam at elevated temperatures was established. The coating consisted of at least two sub-layers, an innermost layer of austenitic phase and a layer of magnetite Fe 3 O 4 . When low loads of steel were used, a third top layer of hematite Fe 2 O 3 was found. The coating provides good protection against atmospheric corrosion, which was significantly better than phosphating.