Toughness and Microstructure of 13Cr4NiMo High-Strength Steel Welds

The microstructures and tensile, Charpy, and crack tip opening displacement (CTOD) properties of 13Cr4NiMo soft martensitic stainless steel flux cored arc welding process (FCAW) weld metals have been studied through different applied postweld heat treatments (PWHT). Phases and microstructural characteristics have been analyzed by scanning electron microscopy (SEM) and x-ray diffraction. The effect of the tempering and double tempering, with and without previous solution annealing, on the impact and fracture toughness has been studied. The role of the retained austenite resulting from tempering has been recognized, and it is suggested that the austenite particles improve the toughness of the welds through their transformation by the transformation-induced plasticity (TRIP) mechanism. microestructures in the as-welded condition; therefore, they


Introduction
or double (high plus intercritical) tempering can be necessary. [9]he solution annealing is performed at temperatures between The poor weldability of martensitic stainless steels, their 950 and 1050 8C.The aim of such treatment is the homogenizasensivity to cold cracking, and the unsatisfactory mechanical tion of the structure by dissolution of the d-ferrite and retained properties in the welded joints led to the development of lowaustenite, which are nonequilibrium solidification products.The carbon (soft) martensitic stainless steels in the 1960s.[1] intercritical tempering at 600 8C produces a softening of mar-If the 12% Cr stainless steels are used as high-strength steels, tensite and a finely dispersed austenite that is stable and nonthey have to be weldable, formable, and have good toughness.[2] transformable during cooling.It is known that such austenite, Hence, in soft martensitic stainless steels, the carbon content which can be observed only by scanning electron microscopy is kept below mass 0.1% to improve weldability by promoting (SEM), increases the toughness, although it slightly reduces a structure with less tendency to cold cracking, better corrosion the strength.resistance, and better toughness.In addition, they have high Regarding the relative use of different welding consumable strength with high toughness and ductility even at very low types for welding structural steel, the consumption of tubular temperatures or thick cross sections.[3,4] Due to low carbon, the wire in the world has markedly increased for the last few years addition of 4 to 6% Ni (the most powerful austenite former and it is expected to continue increasing rapidly.[5,10] Taking after C and N) is required to avoid a significant content of dthis into account, the purpose of the present work is to study ferrite, which is deleterious to both impact and fracture toughthe microstructures resulting from different PWHT applied to ness.For enhanced corrosion resistance together with resistance 13Cr4NiMo weld metals obtained by means of the flux cored to temper embrittlement and tempering, 0.5 to 2% molybdenum arc welding process (FCAW), together with the effects on their is added, depending on the intented use.
tensile, Charpy V-notch impact, and crack tip opening displace-The characteristics of such 13Cr-4NiMo soft martensitic ment (CTOD) properties.In addition, the role of the most stainless steels have resulted in an increasingly worldwide use important phases in the microstructures is considered and correand this trend is expected to continue. [5]The uses of such steels lated with the mechanical properties.are in petrochemical and chemical plants or industries, gas turbine engines, turbine blades, hydraulic turbines, valve bodies, pump bowls, compressor cones and discs, and in a variety of

Experimental
aircraft structural and engine applications. [5,6,7]he weld metals of 13Cr-4NiMo steels have martensitic The FCAW process was used to prepare the weld metals.Multiple-pass welds were performed on AISI 410 plates using The PWHT were intercritical temperings at 600 8C/2 h and accuracy, in the phase abundance results, was calculated from the scale factors and from estimated volume standard deviations.double temperings (670 8C/2 h 1 600 8C/2 h), with and without previous solution annealing at 950 8C/1 h (Table 3).The chemi-Mechanical properties were evaluated by means of tensile, Charpy V-notch impact, Rocwell-C hardness, and microhard-cal composition of the weld metals in different conditions was measured by an optical emission technique (except for C, N, ness tests according to ASTM A 370.The Charpy V-notch impact tests were carried out at 20 and 277 8C.Microhardness O, and S, which were measured by combustion analysis).A scanning electron microscope operated at 25 kV was used to was measured by using a Vickers meter with a load of 20 g and a loading time of 15 s.Fracture toughness was evaluated analyze the microstructures of the weld metals and the fracture surfaces of the specimens for impact and fracture toughness by means of CTOD tests through critical CTOD or maximum load CTOD at room temperature on compact specimens 0.5 in.testing.Samples were ground and electropolished.The electrolyte composition for electropolishing was HClO 4 : 62 mL, etha-thick (1/2 9 T-CT) according to ASTM E-1290.nol: 700 mL, butyl cellusolve: 100 mL, and H 2 O: 137 mL.Then, specimens were etched in Vilella's solution.
To identify austenite and to measure its volume fraction

Results and Discussion
in each weld condition, x-ray diffraction applying a Rietveld analysis was used.The x-ray diffraction patterns were obtained The results of the chemical composition of the weld metals at room temperature with a diffractometer furnished with a in the as-welded condition and with different applied PWHT diffracted beam graphite monochromator.Data were collected are given in Table 4 and show similar compositions for all using Cu K a radiation in the range of 10 # 2u # 120 at a step weld conditions.interval of 0.028.A Rietveld analysis was performed using the The microstructural analysis, tensile properties, hardness, program DBWS-9411. [11]The sample displacement and the Charpy, and CTOD values obtained for all weld conditions are background (modeled with a fifth degree polynomial) were summarized in Table 5. refined independently but not simultaneously, as well as the In the as-welded condition (A), the microstructure is basicly unit cell, the preferred orientation, the Pseudo-Voigt profile martensite with some delta ferrite and some retained austenite parameters, and the scale factor of different phases present in resulting from nonequilibrium solidification (Fig. 1).Intercritithe sample.From the Rietveld analysis, a relative weight fraccal temperings at 600 8C produce the softening of martensitic tion was assigned to the refined phases.This approximation structures and a fine dispersion of precipitated austenite does disregards the contributions of all other minor phases.The not transform during cooling.
In the intercritical tempering conditions (B and C), the austenite contents evaluated by means of Rietveld analysis are seem that after an extensive deformation, microvoids coalesced from austenite/matrix interfaces, as has been observed close to and yield strength.On the other hand, Charpy and CTOD values have had important differences.The CTOD values have had the fracture of Charpy specimens (Fig. 4).
The fracture mode of CTOD specimens was also ductile in all the same trend of the Charpy values (Fig. 8 and 9).From Fig. 8 and 9, it can be noted that conditions C and E, PWHT conditions except for condition B, which was brittle.The fracture mode was also brittle in the as-welded condition A. Typical which have more uniform microstructures without delta ferrite (both due to the solution annealing), have the highest toughness.dimpled-rupture fracture surfaces have been observed by SEM in all weld conditions (Fig. 5), though isolated quasi-cleavage has Between both conditions, Charpy and CTOD values are higher in the double tempering condition E that had a higher austenite been observed in conditions A and B. The quasi-cleavage seems to be associated with the presence of large nonmetallic inclusions content with a more uniform distribution (Fig. 8).Nevertheless, tensile and yield values are higher in condition C (Fig. 9).(Fig. 6(a) and (b)), since a larger particle diameter makes the nucleation of a cleavage crack easier.
Tensile and yield strength, Charpy-V notch impact energy, and CTOD values of welds with different applied PWHT are summarized in Fig. 7. On one hand, from this figure, it can be seen that welds have had moderate differences regarding tensile   To evaluate a mechanical transformation of the austenite tensile specimens were electroplated with chromium so that metallographic examination could be performed exactly close particles, x-ray diffraction analysis has been carried out on the regions close to the fracture surfaces of the tensile, Charpy, to the fractures (Fig. 4, 10, and 11).From these figures can be noted an important plastic flow, extensive deformation of the and CTOD specimens of condition E, where intense deformation fields have been observed.Thus, irregular fracture surfaces austenite particles, and microvoids around both austenite particles and nonmetallic inclusions.These regions have been ana-were slightly ground and electropolished to prepare smooth surfaces for x-ray diffraction analysis.show diffraction patterns of these three regions and the typical diffraction pattern of regions far from the fractures of the three kinds of test specimens.From these figures, it can be seen that austenite is absent in regions close to the fractures and only the ferritic phase is present.Nevertheless, both phases appear far from the fractures.
The absence of austenite in regions immediately close to the fractures together with the hardness increase of these zones plastically deformed may indicate that austenite particles have been transformed into martensite by transformation-induced plasticity (TRIP).It could be suggested that the occurrence of austenite TRIP phenomena in regions with intense deformation fields led to an increased ductility, impact, and fracture toughness of this alloy with finely dispersed austenite.In this last sense, the TRIP mechanism in the plastic zone around a front fracture could absorb energy, effectively contributing to the enhancing of toughness.

Conclusions
• Postweld heat treatments have produced the softening of the microstructures together with a finely dispersed austenite precipitation.In the cases of solution annealing previous • Higher values of CTOD, Charpy, and ductility have been annealing plus double tempering PWHT, which produced obtained by means of microstructures composed by temthe maximum softening with the highest ductility and pered martensite and austenite, without delta ferrite.The toughness.The decreasing of the yield and tensile strength highest values were obtained from the solution annealing for all applied PWHT, regarding the as-welded condition, plus double tempering condition, where the austenite conhas not been relevant compared to the increase in ductility tent was higher and more uniformly distributed.and toughness.• The yield and tensile strength values had slight differences with the applied PWHT.They were lower for the solution • The study of the microstructures close to the fractures of tensile, Charpy, and CTOD specimens showed the mechani-(CONICET-UNLP) Institute, for assistance in the SEM studies and to Eduardo Benotti for preparing and precracking the cal transformation of the austenite particles.It is suggested that the beneficial effect of the dispersed austenite on the CTOD specimens.toughness and ductility of this alloy is based on the localized TRIP mechanism development.

Fig. 7
Fig. 7 Tensile and yield strength, Charpy V-notch impact energy, and CTOD values of different welds.A: as-welded condition, and B through E: with different applied PWHT Figure 12(a) through (d) lyzed by means of microindentation hardness testing.The

Fig. 11
Fig. 11 Microstructure observed by SEM close to the fracture of tensile specimens.Microvoids coalesced from austenite/matrix interfaces and from nonmetallic inclusions

Table 1 Nominal composition of the consumable (wt
.%) similar.Values are also similar in the double tempering conditions (D and E) (670 8C/2 h 1 600 8C/2 h), but they are about

Table 2 Welding parameters The
fracture modes of Charpy specimens were ductile in all Current InterpassPWHT conditions.Typical dimpled-rupture fracture surfaces

Table 3 Applied postweld heat treatments
610-Volume 9(6) December 2000 Journal of Materials Engineering and Performance