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Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading
Journal Pre-proof Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading Sudhir Kumar, Prakriti Kumar Ghosh PII:
S0921-5093(19)31520-5
DOI:
https://doi.org/10.1016/j.msea.2019.138734
Reference:
MSA 138734
To appear in:
Materials Science & Engineering A
Received Date: 2 October 2019 Revised Date:
24 November 2019
Accepted Date: 25 November 2019
Please cite this article as: S. Kumar, P.K. Ghosh, Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading, Materials Science & Engineering A (2019), doi: https://doi.org/10.1016/j.msea.2019.138734. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section
Sudhir Kumar: Conceptualization, Methodology, Data curation, WritingOriginal draft preparation, Validation, Investigation PK Ghosh: Investigation, Supervision, Reviewing and Editing,
Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading Sudhir Kumara*, Prakriti Kumar Ghosha a
Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Uttarakhand-247667, India
b
School of Engineering and technology, Central University of Haryana, Haryana-123031 * Corresponding author: E-mail address- [email protected]
ABSTRACT Surface modification of medium carbon AISI 4340 structural steel using TIG arcing process under various heat input (Ω) of varying arcing current (I) and arc travel speed (S) has been studied. Weld isotherm and thermal cycle of the fused zone has been analytically established and validated with experimentally measured results. TIG arcing process has been found to change the microstructural characteristics of the fusion modified matrix by transformation of fine niddle shape martensite, which has significantly enhanced its hardness from 256±10 HV to 745±8 HV and thus its mechanical properties. Formation of residual stresses in the surface modified the substrate is examined by hole drill method up to a depth of 1.3 mm. A residual stress of tensile nature is found to exist up to an appreciable depth from the top modified of the substrate, which has adversely affected its static and dynamic properties by enhancement of resolved stress in the matrix during the bend test. Surface modification improves flexural properties of the substrate with a maximum improvement of flexural yield strength at a relatively slow arc travel speed of 6 cm/min, while the Ω lies in the range of 0.6-1.21 kJ/mm. A favourable to improvement in static and dynamic properties of the substrate were found at relatively higher Ω of TIG arcing at slower S. It has primarily happened due to development of relatively slower cooling rate in the matrix introducing comparatively ductile lath martensite in it. GRAPHICAL ABSTRACT
1
Keywords: TIG arcing, AISI 4340 steel, isotherm, thermal cycle, microstructure, hardness, three point bend test, static and dynamic loading, residual stresses. 1. Introduction Surface of a component often requires unique combination of improved surface properties like hardness, resistance to wear and corrosion and residual stresses for its better service life. These properties can be improved by surface modification technique. The surface of a metallic substrate can be modified by four different technics: - 1) Deposition of material of desired chemistry [1], 2) Changing of surface chemistry [2,3], 3) Modification of surface microstructure [4–6] and 4) Mechanical surface treatment [7]. Based on these ideas number of surface modification techniques are adopted in industries. Surface modification by commonly used thermal techniques like carburizing, nitriding and thermal/plasma spray coating are having number of technical difficulties. Primarily they produce relatively narrow thickness of modified zone and are also difficult to apply on large components of gear, spindles, connecting road, large rollers, etc. with homogeneous quality of modification at uniform zone thickness [8]. Further these processes are uneconomical to apply for modification of small section or area in limited quantity. In case of flame hardening of components like cams, leavers and push roads, it is difficult to control overheating of the component due to relatively low control of heat spread of the flame while during induction hardening it is difficult to do surface modification of complex shape work piece [9]. To overcome such problems, surface modification has been successfully carried out by melting of substrate surface using concentrated point heat source like laser and electron beam [10]. But it is realized that these processes are also having some limitations. They are primarily found as non-versatile for application at any site of the job and uneconomical for general purpose use due to large overhead expenditure. Further they produce relatively shallow depth and width of surface modification per pass [11]. 2
In this context, Tungsten Inert Gas (TIG) arcing is found quit useful for surface modification of metals and alloys. The TIG arcing process have several advantages such as relatively high precision control, low health hazards, less processing time and appreciable versatility in application at comparatively low cost over other modification process as stated above [12,13]. The TIG arcing process have the capability to modify the surface in greater depth (ranging in mm) as compared to carburizing and nitriding [12]. Such superior depth of modification is made possible by applying instant intense heat by arcing over the surface, which results a quick localised surface melting. It establishes a large thermal gradient due to relatively strong effective heat sink around the fused zone similar to laser and electron beam surfacing [14]. Here a critical control of thermal gradient gives desired microstructure by dictating phase transformation in the fusion zone (FZ). Further the size and shape of the fused zone (FZ) are controlled by heat input (Ω) under combinations of TIG parameters (current, voltage and travel speed) where, argon gas used as shielding. A through study on microstructure and hardness behaviour of the fused zone (FZ) and heat affected zone (HAZ) of surface modified AISI 4340 steel conducted by sudhir et. al. [15]. Consequently the effect of TIG arcing on tensile and tensile-fatigue properties is critically examined and discussed in a previous article [16]. It is observed that the TIG arcing significantly improves the surface hardness in FZ up to an appreciable depth at optimum process parameter. It is also understood that the TIG arcing may introduce residual stresses on the modified surface which has appreciable influence on its mechanical properties, that should be more thoroughly understood for mechanical characterization of the matrix. In consideration of the above observations, an effort was done to understand thoroughly the effect of TIG arcing behaviour on AISI 4340 steel substrate. The “three dimensional double ellipsoidal heat source model” proposed by Nguyen et al. (1999) [17] is used to validate the experimental work in the context of the understanding on the bead geometry and its thermal cycle. Effect of thermal behaviour of the modified matrix, affecting its microstructure and residual stress generation, on the uniaxial bending properties of the surface modified substrate under static and dynamic loading has been studied. 2.
ANALYTICAL ESTIMATION OF ARC BEHAVIOUR The estimation given by Nguyen et al. (1999) [17] for the analysis of a double
ellipsoidal moving point heat source (Fig. 1) of arc welding. In view of that this model has been used for analytical estimation of isotherms and thermal cycle produced by controlled
3
fusion using TIG arcing process [14]. The analytical approach followed in this exercise has been stated below.
Fig.1 Graphical representation of analytical model of heat source [14]. At a location (x,y,z) in semi-infinite plate, the temperature Td can be estimated as follows [14].
dt' . ' 2 ' 2 1 2 a (t t ) + a . 1 2 a (t t ) + b h h 3 3 .Q A W t Td = ∫ 2 ρ .c .π π 0 A' B' + 1 2 a (t - t ' ) + c h2 f 1 2 a (t - t ' ) + c h2 b
(
) (
(
)
)
(
)
+ T .......(1) 0
Where, QAW is the arc heat transferred to the weld pool and estimated by equation 2. QAW = η.V .I Joule .........................(2) 3(x - v.t ' ) 2 3y 2 3z 2 A ' = rf .exp ......................(3) ' 2 ' 2 ' 2 12a(t - t ) + c hf 12a(t - t ) + a h 12a(t - t ) + b h 3(x - v.t ' ) 2 3y 2 3z 2 B' = rb .exp ..........................(4) ' 2 ' 2 ' 2 12a(t - t ) + c hb 12a(t - t ) + a h 12a(t - t ) + bh
Where, rf (front) and rb (behind) are the proportion coefficients of heat source, calculated as rf =
2.c hf
( c hf
+ c hb )
...........................(5)
4
rb =
2.c hb
and
( c hf
+ c hb )
.............................(6)
-
η is arc efficiency considered as 75%,
-
Arc voltage is denoted by V and arcing current is denoted by I.
-
The material constants mass density(ρ) , specific heat(c) and thermal diffusivity (a).
-
ah, bh, chf and chb are the ellipsoidal parameters as shown in fig. 1. The heat source parameters chf and chb are knowing as chf = ah and chb = 2 chf [18].
3.
EXPERIMENTAL
3.1
Surface modification AISI 4340 steel plate having dimension of 150×75×10 mm in length, width and
thickness respectively was used in this investigation. Chemical wt% in medium carbon AISI 4340 steel is shown in Table-1. It is also known as Ni-Cr-Mo steel on the basis of its alloying content. Prior to treatment the substrate surface was cleaned by using surface grinding followed by acetone cleaning. The process was carried out by non-consumable tungsten electrode having 3.2 mm diameter. Argon as a shielding gas (15 l/min) was used for TIG arcing process. The primary parameters (I and S) are affecting the surface modification by their significant influence on Ω of TIG arcing process, they are varied I from 80 to 140 A and S from 6 to 15 cm/min and studied their influence on characteristics of the modified matrix. Ω of the TIG arcing process was estimated as follows[19]. Ω = [(η×I×V×60)/(S×1000)]×η
------------- (6)
Where, Ω is heat input (kJ/mm) -
I considered as arcing current
-
S considered as arc travel speed (mm/min)
-
η considered as arc efficiency (0.75)
-
V considered as arc voltage (V= 10.5±1.0 V).
The temperature and thermal cycle of at a desired locations corresponding to the FZ and HAZ has been measured by R- type thermocouple.
5
Table 1. Chemical composition of AISI 4340 steel. C
Cr
Ni
Mo
Mn
S
P
Si
Fe
0.39
0.79
1.65
0.22
0.71
0.04
0.04
0.21
remaining
3.2
Microstructure Microstructure of the substrate were studied on their transverse section by preparing
them using standard metallographic procedure followed by etching in alcoholic 5% nitric acid solution (nitric acid 5% and remaining part is ethanol). Macrograph of the profile of the fused modified region and its microstructure was studied under optical microscope. The profile of the FZ and HAZ was measured under optical macroscope by placing the specimen on a sliding table capable to move in the direction of X and Y coordinate with the help of in built screw gauge micrometre.
3.3
Mechanical testing Hardness measurement of modified substrate was carried out on its metallographically
prepared and etched transverse section using Vickers micro-hardness tester at a load of 300 grams with 30 s dwell time A 0.5 mm distance from its top surface was taken for hardness measurement in FZ and for the same of the HAZ the indentation was made in the matrix at 0.3 mm from the fusion line. The hardness of surface modified substrate was studied at the low and high Ω of 0.3 to 0.6 kJ/mm respectively. Similarly hardness of the base metal was also studied and compared. Mechanical tests were performed on INSTRON Servo hydraulic machine with load cell limit of ± 50 kN and ±75 mm cross head movement. Tests were performed at room temperature. Schematic diagram of the specimen and the loading system has been shown in Fig. 2, where modified zone experiences maximum tensile stress with load applied at the centre. The sample size of both the tests was kept as 150×W×9 mm for length, width and thickness respectively where, W varied with modified bead width. ASTM E-855-08 “Standard Test Method for Bend Testing of Metallic Flat Materials” [20] was used with a strain rate of 1.0 mm/min. Average result of three samples were reported. Estimation of stresses under the three-point bending load was also done according to ASTM E-855-08. Flexural stresses (σ) in MPa were calculated by eq. 7, in which, M- bending moment (N.mm),
6
Y- perpendicular distance from neutral axis (mm) and Z considered as moment of inertia (mm4). σ = M*Y/Z
---------- (7)
M and I are estimated as M = F*L/4
--------- (8)
I = W*T3/12
---------- (9)
Where, -
F is force applied (N).
-
L, W, and T are sample dimension as shown in fig. 2. The fatigue test was done at 10 Hz frequency. Stress ratio (R) is taken as 0.1. where R
is the ratio of minimum (σmin)and maximum (σmax) applied stresses, respectively. The endurance limit was considered as 1×106 cycle after that the test was stop by considering no failure in the sample. Representation of fatigue test results was compliance with ASTM E739 “Standard Practice for Statistical Analysis of Linearized Stress-Life (S-N) Fatigue Data” [21]. In case of failure the fracture surface was studied under scanning electron microscope.
Fig. 2 Static and dynamic three point bend test sample for Single-pass surface modification. 3.4
Residual stress analysis 7
ASTM E-837 “Standard Test Method for Determining Residual Stresses by the HoleDrilling Strain Gage Method” [22] was used. A strain gauge rosette (FRS-2-17) [23] was used. In a strain gauge rosette three strain gauges of the rosette, located at 00, 900 and 2250 in radial direction as shown in Fig. 3, are used to find out the strain and stresses in different orientations. The residual stresses in different direction were estimated by using eqs. given below. Longitudinal direction = P – Q
-------
Transverse direction = P + Q
-------
(10) (11)
Where, -------
1
-------
(13)
-
ε1, ε2 and ε3 are the measured strain.
-
E is Modulus of Elasticity (210 GPa) and ν is Poisson’s ratio (0.29) respectively considered as material constants.
-
ā and
are calibration coefficients [16].
Fig. 3 A systematic view of residual stress measurement setup [16].
4.
(12)
RESULTS AND DISCUSSIONS
8
4.1.
Geometry of modified zones Typical macroscopic cross sectional view of the TIG modified surface by changing I
from 80 to 140 A at constant V and S of 10.5±1.0 V and 9 cm/min respectively as shown in Fig. 4. The figure shows that the area of fusion modified zone varies significantly with the change in process parameters affecting the Ω.
Fig. 4 Typical macroscopic cross sectional view of modified region. The geometry of the modified zone has been characterized by four different factors such as 1. Depth of penetration (Pd), 2. Bead width (Wb), 3. Width of HAZ (Whaz) and 4. Area of FZ (AFZ). At the given V of 10.5±1.0 V the effect of variation in I and S on the WB, Pd, Whaz and AFZ has been shown in Figs. 5 (a-d) respectively. The figures show that the increase in I from 80 to 160A significantly increases the Wb, Pd, Whaz and AFZ. It has been further observed that the WB, Pd, Whaz and AFZ increase appreciably with the reduction of S. Such a variation in WB, Pd, Whaz and AFZ as a function of I and S may be primarily attributed to the variation in Ω to FZ governing the size and fluidity of molten pool [24]. The empirical correlations of the WB, Pd, Whaz and AFZ as a function of I and S depicted in Figs. 5 (a-d) respectively have been given below.
W B = − 0.12213 S + 0.04532 I + 9.17 × 10 −5 SI + 0.5204
……
(14)
Pd = − 0.0172 S + 0.02617 I − 5.48 × 10 −4 SI − 0.2314
……
(15)
Whaz = −0.71S + 1.04 ×10−2 I + 7.33 ×10−6 IS + 0.956
……
(16)
AFZ = 0.496S + 0.252I − 1.09 ×10−2 IS − 13.11
……
(17)
9
Fig. 5 Effect of I and S on (a) Pd, (b) Wb (c) Whaz and (d) AFZ at an V of 10.5 ± 1.0 V. 4.2 Thermal characteristics of the modified zone Thermal characteristics of the fusion zone is critically controlled by the weld pool due to their influence on contact area with the conductive substrate and exposed area for other types of heat dissipation as well as heat content of fused mass. Thus, it affects solidification mechanism of FZ and consequently influences the microstructure and properties of the solidified matrix. The shape and size of FZ and HAZ controlled by the Ω through variation of I, V and S may be clearly understood from the isotherm of fused zone of the substrate. The isothermal curves are estimated by the heat flow expression (eq. 1) as discussed earlier. The size of FZ and HAZ is illustrated by the isotherm at different Ω as a function of I, V and S. The isothermal curves in both the planes of x-y and y-z, thermal cycle and cooling rate are estimated by the expression given in chapter 2 equation 1. 4.2.1 Isothermal curves in different planes
10
At a given V of 10.5±1.0 V the effect of variation in I (80 to 140 A) and S (6 to 15 cm/min), on FZ isotherm in XY plane on substrate surface has been shown in Figs. 6 (a-b). Similarly, FZ isotherm in YZ plane on transverse to the direction of arc travel is shown in Figs. 7 (a-b). The Fig. 6 (a) depicts that the increase of I increases the width and length of isotherm at a given S, whereas at a given arc current the increase in S significantly decreases the width and length of the isotherm as shown in Fig. 6 (b). Similarly the Fig. 7 (a-b) depicts that the size of isotherm of modified zone increases with increase of I. This is primarily attributed to the increment in heat transfer to the FZ with the increase of I at a given S. The figures also reveal that the increase of S at a given I and V reduces the width, length and depth of the isotherm. This is primarily happened at decrease in heat transfer per unit length by increase in speed of arc travel. The increment in S at a given I reduces the energy input per unit length, which influences the distribution of energy into the work piece. The increment in S also reduces the extent of distribution of energy into the substrate. The Fig. 7 (a-b) further depicts a comparison of isotherm of the modified zone at I (80 to 140 A) and S (6 to 15 cm/min). The geometry of modified zones of substrate, defined as the fusion zone and HAZ in concurrence to their estimated zone of peak temperatures of 1700 K and 1073 K respectively, are measured as a signature of relevant change of microstructure in the matrix. The figure shows that the predicted geometry (broken line) of modified zone is quite close to their experimental observations (continuous line) with a difference of 4 ± 2 %.
11
Fig. 6 The estimated isotherms in XY plane at given V 10.5±1.0 V of (a) varied I of 80, 100, 120 and 140 A and (b) varied S to 6, 9, 12 and 15 cm/min.
12
Fig. 7 Comparison of estimated and measured isotherms in YZ plane at given V 10.5±1.0 V Effect of (a) varied I to 80, 100, 120 and 140 A and (b) varied S to 6, 9, 12 and 15 cm/min.
13
4.2.2
Thermal Cycle Thermal cycle shows the nature of variation in temperature during heating and
cooling with respect to time in FZ and HAZ. At a given range of Ω of 0.3 – 0.60 kJ/mm, the typical thermal cycle of FZ and HAZ developed at I as 80 and 140 A has been shown in Figs. 8 (a-b) while the V and S were kept constant as 10.5±1.0 V and 12 cm/min. The figure reveals a close approximation of the measured and estimated thermal cycle at different depth of Z axis (Fig. 1) relevant to the FZ and HAZ of different arcing process parameters. It also depicts that the increase of Ω by increase in I reduces the cooling rate (CR). Such changes were occurred due to reduction in area of effective heat sink of the substrate and lower temperature gradient for heat transfer from the relatively larger fusion zone and wider HAZ at higher Ω. This has caused a reduction in cooling rate from 800 to 500◦C [25,26].
Fig. 8 Thermal cycle at different depth of Z axis correspond to the FZ and HAZ at the I and Ω of (a) 80 A; 0.30 kJ/mm respectively and (b) 140 A; 0.60 kJ/cm respectively.
14
Fig. 9 FZ and HAZ cooling rate in inter-critical temperature range (800-5000C). A comparison of measured cooling rate and estimated cooling rate at the inter-critical temperature range (800-5000C) of FZ and HAZ obtained from respective thermal cycle plots of different I and Ω has shown in fig. 9. The figure reveals that as Ω increase by increase of I leads to decrease in cooling rate in FZ as well as HAZ. It was also noticed that the estimated cooling rate gives a close approximation with measured cooling rate. The final phase transformation of the FZ and HAZ was primarily governed with the changes in isotherm and cooling rate . 4.3
Microstructure Proeutectoid ferrite and pearlite were observed in microstructure of the substrate as
shown in Fig. 10 and having hardness of 256 ± 10 HV. Typical microstructures of the fusion modified zone prepared at relatively low (0.3 kJ/mm) and high (0.6 kJ/mm) Ω of varying arc current to 80 and 140 A employed at a given S and V of 12 cm/min and 10.5±1.0 V respectively has been shown in Fig. 11(a(i) and b(i)) respectively. The micrograph presented in Fig. 11 (a(i)) reveals the significant transformation of needle shape martensite beside acicular ferrite. However, as the Ω is increased from 0.30 to 0.60 kJ/mm, the transformation is predominated by the formation of lath martensite in the matrix containing acicular and patches of ferrite as shown in Fig. 11 (b(i)). Such transformation was critically attributed to cooling rate, which is controlled by the arcing parameter as discussed above. Similarly, phase transformation observed in the HAZ of base material at Ω 0.3 and 0.6 kJ/mm has been shown in the microstructures presented in Figs. 11 (a(ii)) and (b(ii)) respectively. The Fig. 11
15
a(ii) reveals that martensite along with bainite and allotriomorphic Ferrite in the matrix was seen at the Ω of 0.3 kJ/mm, as the Ω increase to 0.6 kJ/mm, the martensitic transformation was supressed and relative amount of bainite and pearlite transformation in the matrix. It was also noticed that the grain coarsening were observed as the Ω increase from 0.30 to 0.60 kJ/mm shown in figs. 11 a(ii) and b(ii).
Fig. 10 Microstructure of the AISI 4340 steel substrate
Fig. 11 Microstructures of (i) FZ and (ii) HAZ under varied Ω of different I a) 0.3 kJ/mm, I= 80 A and b) 0.6 kJ/mm, I= 140 A in modified surface at constant S and V of 12 cm/min and 10.5 ± 1.0 V respectively.
16
The significant increase of hardness of the substrate from 256±10 HV confirms the observed change in microstructure in its FZ and HAZ with a change of Ω from 0.3 to 0.6 kJ/mm as discussed above. With increase of Ω relatively low to high, the hardness of the FZ of the medium carbon substrate reduces from 745±8 HV to 650±16 HV, while HAZ hardness reduces from 567±12 HV to 435±22 HV. Such changes in hardness of FZ and HAZ related to Ω, that may have primarily caused by microstructural transformation from fine niddle shape martensite to lath martensite in the fusion zone and relatively coarser martensite to bainite in HAZ respectively due to reduction in cooling rate as stated above. 4.3 Flexural properties The flexural yield strength and maximum strength of as received AISI 4340 structural steel under bending load (Fig. 2) are found as about 540 and 1160 MPa respectively. At different S of 6, 9, 12 and 15 cm/min, the variation in flexural properties of surface modified substrate prepared at Ω in the range of 0.24 to 1.21 kJ/mm by changing the I has been shown Figs. 12 (a-d) respectively. It is observed that the single-pass TIG arc surfacing significantly affects the bend properties such as flexural properties of a component as shown in Figs. 13 (a-b). The figures reveal that a low Ω below 0.4 kJ/mm often shows a tendency to maximum lowering of the flexural properties as compared to base metal. This has happened due to increase of cooling rate at low Ω of weld pool, promoting hard phase transformation in it. This is found as formation of needle shape martensite in the matrix, which has significantly low ductility that lead to sudden failure. Formation of residual stress due to such hard phase transformation may have also put an adverse effect on flexural yield strength. Further, it is observed that as the Ω is increased by decreasing S from 12 cm/min to 6 cm/min, there is an appreciable increase in flexural yield strength as well as flexural maximum stress and flexural extension as shown in Fig. 12 (a-c). Such behaviour has happened primarily due to relatively softer lath martensite transformation in the matrix at relatively slower cooling rate of higher Ω (Fig. 9). The Fig. 13 (a) shows that at the S of 6 cm/min, there is maximum improvement in flexural yield strength of the order of 25 to 35 % with respect to base metal observed at approximately similar maximum flexural stress. Whereas, maximum reduction in flexural properties is observed at an I and S of 80 A and 15 cm/min respectively.
17
Fig. 12 Flexural stress-extension plots by different Ω by changing I at different S of (a) 6, (b) 9, (c) 12 and (d) 15 cm/min. Typical fracture surface of the specimen failed under three point bend test has been shown in Fig. 14. The fracture surface reveals the occurrence of brittle fracture in the modified zone followed by ductile fracture in base material primarily due to presence of relatively brittle phase in the fusion modified matrix. Micro cracks were observed at the centre of the modified zone, which may have generated by the presence of high stresses that may cause failure in the plane parallel to that of the applied load.
18
Fig. 13 An effect of different I and S on a) Flexural yield strength, b) flexural maximum strength, observed during three point bend test.
Fig. 14 Macro and micro fractographs of samples failed under bending load of flexural test. 4.4
Fatigue properties under bending load
19
Fatigue life of the base material and modified surface under bending load has shown in Figs. 15 (a-d). The figure reveals the fatigue life of the modified surface has been significantly decreased as compare to base material. It is observed that at the Ω in the limit of 0.24 to 0.32 kJ/mm the endurance limit decreases from σmax of 480 to 405±5 MPa with respect to the base metal. However, Fig. 15 (a-d) also depicts that with the increase of Ω from 0.24 to 1.21 kJ/mm, an improvement in σmax from about 400 to 500 MPa at the endurance limit of modified substrate was observed. Such improvement is in agreement to the flexural strength and microstructural changes under the surface modification as previously discussed. At a constant V and S of 10.5±1.0 V and 6 cm/min respectively, an approximately same endurance limit with respect to the base material (σmax of 480 MPa) is observed when the Ω lies in the range of 0.99 - 1.21 kJ/mm. But there is a reduction in finite fatigue life as shown in Fig. 15 (a). Hard phase transformation in the matrix is attributed to change in slope of S-N curve, which is relatively reduced from that of the base metal.
Fig. 15 The behaviour of surface modified steel S-N curve by changing I and S of (a) 6, (b) 9, (c) 12 and (d) 15 cm/min.
4.5 Residual stresses
20
Residual stresses are formed due to differential thermal gradient and microstructure modification of the substrate during arcing in FZ [27–29]. At different depth of the substrate in reference to its modified surface, the variation of residual stresses with respect to direction of the weld bead has shown in Figs. 16 (a and b). The figures show that increase in Ω enhances the residual stresses at every location of the matrix of the modified substrate. However, the Fig. 16 (a) depicts that the increase of Ω from 0.24 to 1.21 kJ/mm appreciably enhances the longitudinal residual stress in the matrix close to top surface of the substrate to about 100 and 160 MPa respectively followed by a significant reduction to compressive stress of about −225 and −25 MPa respectively at depth of about 0.4 mm from the surface. Here it is interestingly noted that the reduction of residual stress is significantly higher in case of the lower Ω with respect to that marked at higher Ω. The Fig. 16 (b) shows that the transverse residual stresses in the matrix close to the top modified surface of the substrate increases from about 210 to 315 MPa with the increase of Ω from 0.24 to 1.21 kJ/mm. But magnitude of stresses continuously reduces significantly with the increase of depth from the surface. Here it is interestingly observed that up to a depth of about 0.2 mm the residual stresses present in the matrix are of tensile nature whereas at a larger depth beyond about 0.25 mm the residual stresses show a tendency to be of compressive nature, especially at the lower Ω of 0.24 kJ/mm. The presence of tensile stresses in both directions at top surface of the modified zone may have a significant adverse effect on fatigue properties during application of unidirectional bending load (Fig. 2). It may have primarily happened due to additional support of residual stresses to the applied stress for enhancement of resolved stress [30].
21
Fig. 16 At a given arc parameter a) Variation in longitudinal stresses b) Variation transverse stresses. The resolved stresses for three point flexural bend stresses were calculated by the eq. 18. i =5
σ resolved = ∑ σ a + σ r i =−5
i
i
……………………………..……. (18)
Where σa is applied stress due to load, σr is residual stresses due to surface modification and i is thickness of the flexural sample (about 10 mm thickness). Here i=0 mm means neutral axis of the sample. The modified surface resolved stress at lower fibre under the flexural test of single-pass TIG arc modified sample prepared at the Ω of 0.24 to 1.21 kJ/mm is shown in Fig. 17. The resolved stress was reached to about 725 MPa up to a depth of 0.15 mm followed by a sudden reduction to 370 MPa, when the applied stress of testing was 600 MPa. During flexural test, availability of tensile residual stresses play a relatively more prominent role for crack initiation and propagation that adversely affects the flexural properties in static as well as dynamic loading.
Fig. 17 Effect of residual stresses on resolved stress under bending load. 22
5. Conclusions •
Thermal characteristics of TIG arcing such as isotherm, thermal cycle and cooling rate of AISI 4340 steel has been established with the help of an analytical model and experimental measurement.
•
Analytical results are found closely in agreement of the order of 94% to the measured values. The three point bend properties of the substrate under the static and dynamic loading are significantly affected by TIG arc surface modification, which is primarily attributed to development of residual stresses and its microstructural modification.
•
Surface modification has improved the flexural properties of the substrate with a maximum improvement of flexural yield strength of the order of 25 to 35 % during processing at relatively slow S of 6 cm/min, while the Ω lies in the range of 0.6-1.21 kJ/mm. -
This has primarily happened because the increase of cooling rate at higher S promotes transformation of fine niddle shape martensite in the fusion modified matrix that significantly enhances its hardness to 745±8 HV with a decrease in ductility and consequently reduces the bending properties under the static and dynamic loading.
•
The increase of Ω from 0.24 to 1.21 kJ/mm improves σmax from about 400 to 500 MPa at the endurance limit of fatigue life of the surface modified substrate. Further development of more residual stresses of tensile nature at the surface modified at higher Ω adversely affect the mechanical properties under both the static and dynamic bending load due to enhancement of resolved stresses in it by additional support to the applied stress.
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[10] E. Mohseni, E. Zalnezhad, A.A.D. Sarhan, A.R. Bushroa, A Study on Surface Modification of Al7075-T6 Alloy against Fretting Fatigue Phenomenon, Adv. Mater. Sci. Eng. 2014 (2014) 1–17. doi:10.1155/2014/474723. [11] T. Brockhoff, E. Brinksmeier, Grind-Hardening: A Comprehensive View, CIRP Ann. 48 (1999) 255–260. doi:10.1016/S0007-8506(07)63178-3. [12] P.K. Ghosh, R. Kumar, Surface Modification of Micro-Alloyed High-Strength LowAlloy Steel by Controlled TIG Arcing Process, Metall. Mater. Trans. A. 46 (2015) 831–842. doi:10.1007/s11661-014-2670-x. [13] P. Patel, S. Mridha, T.N. Baker, Influence of shielding gases on preheat produced in surface coatings incorporating SiC particulates into microalloy steel using TIG
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e:%5CBIBLIO%5CArticles%5CSolution_transient-temp-3D-moving-heatsource_Nguyen_1999.pdf. [18] V.K. Goyal, P.K. Ghosh, J.S. Saini, Analytical studies on thermal behaviour and geometry of weld pool in pulsed current gas metal arc welding, J. Mater. Process. Technol. 209 (2009) 1318–1336. doi:10.1016/j.jmatprotec.2008.03.035. [19] C. Pandey, A. Giri, M.M. Mahapatra, On the prediction of effect of direction of welding on bead geometry and residual deformation of double-sided fillet welds, Int. J. Steel Struct. 16 (2016) 333–345. doi:10.1007/s13296-016-6007-z. [20] ASTM – E855-08, Standard Test Methods for Bend Testing of Metallic Flat Materials for Spring Applications Involving Static Loading, Annu. B. ASTM Stand. 03.01 (2013) 1–8. doi:10.1520/E0855-08R13. [21] ASTM, Standard Practice for Statistical Analysis of Linear or Linearized Stress-Life ( S-N ) and Strain-Life ( ´ -N ) Fatigue Data 1, Annu. B. ASTM Stand. i (2012) 1–7. doi:10.1520/E0739-10.2. [22] ASTM, Determining Residual Stresses by the Hole-Drilling Strain-Gage Method,
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26
Date- 02, October 2019 SUBJECT: Declarations of interest
Dear Editor I am enclosing herewith a manuscript entitled “Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading” for publication in “Materials Science and Engineering: A” for possible evaluation. I also like to state that the “Declarations of interest: none” for this research article.
Sudhir Kumar
S0921-5093(19)31520-5
DOI:
https://doi.org/10.1016/j.msea.2019.138734
Reference:
MSA 138734
To appear in:
Materials Science & Engineering A
Received Date: 2 October 2019 Revised Date:
24 November 2019
Accepted Date: 25 November 2019
Please cite this article as: S. Kumar, P.K. Ghosh, Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading, Materials Science & Engineering A (2019), doi: https://doi.org/10.1016/j.msea.2019.138734. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Author Contributions Section
Sudhir Kumar: Conceptualization, Methodology, Data curation, WritingOriginal draft preparation, Validation, Investigation PK Ghosh: Investigation, Supervision, Reviewing and Editing,
Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading Sudhir Kumara*, Prakriti Kumar Ghosha a
Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Uttarakhand-247667, India
b
School of Engineering and technology, Central University of Haryana, Haryana-123031 * Corresponding author: E-mail address- [email protected]
ABSTRACT Surface modification of medium carbon AISI 4340 structural steel using TIG arcing process under various heat input (Ω) of varying arcing current (I) and arc travel speed (S) has been studied. Weld isotherm and thermal cycle of the fused zone has been analytically established and validated with experimentally measured results. TIG arcing process has been found to change the microstructural characteristics of the fusion modified matrix by transformation of fine niddle shape martensite, which has significantly enhanced its hardness from 256±10 HV to 745±8 HV and thus its mechanical properties. Formation of residual stresses in the surface modified the substrate is examined by hole drill method up to a depth of 1.3 mm. A residual stress of tensile nature is found to exist up to an appreciable depth from the top modified of the substrate, which has adversely affected its static and dynamic properties by enhancement of resolved stress in the matrix during the bend test. Surface modification improves flexural properties of the substrate with a maximum improvement of flexural yield strength at a relatively slow arc travel speed of 6 cm/min, while the Ω lies in the range of 0.6-1.21 kJ/mm. A favourable to improvement in static and dynamic properties of the substrate were found at relatively higher Ω of TIG arcing at slower S. It has primarily happened due to development of relatively slower cooling rate in the matrix introducing comparatively ductile lath martensite in it. GRAPHICAL ABSTRACT
1
Keywords: TIG arcing, AISI 4340 steel, isotherm, thermal cycle, microstructure, hardness, three point bend test, static and dynamic loading, residual stresses. 1. Introduction Surface of a component often requires unique combination of improved surface properties like hardness, resistance to wear and corrosion and residual stresses for its better service life. These properties can be improved by surface modification technique. The surface of a metallic substrate can be modified by four different technics: - 1) Deposition of material of desired chemistry [1], 2) Changing of surface chemistry [2,3], 3) Modification of surface microstructure [4–6] and 4) Mechanical surface treatment [7]. Based on these ideas number of surface modification techniques are adopted in industries. Surface modification by commonly used thermal techniques like carburizing, nitriding and thermal/plasma spray coating are having number of technical difficulties. Primarily they produce relatively narrow thickness of modified zone and are also difficult to apply on large components of gear, spindles, connecting road, large rollers, etc. with homogeneous quality of modification at uniform zone thickness [8]. Further these processes are uneconomical to apply for modification of small section or area in limited quantity. In case of flame hardening of components like cams, leavers and push roads, it is difficult to control overheating of the component due to relatively low control of heat spread of the flame while during induction hardening it is difficult to do surface modification of complex shape work piece [9]. To overcome such problems, surface modification has been successfully carried out by melting of substrate surface using concentrated point heat source like laser and electron beam [10]. But it is realized that these processes are also having some limitations. They are primarily found as non-versatile for application at any site of the job and uneconomical for general purpose use due to large overhead expenditure. Further they produce relatively shallow depth and width of surface modification per pass [11]. 2
In this context, Tungsten Inert Gas (TIG) arcing is found quit useful for surface modification of metals and alloys. The TIG arcing process have several advantages such as relatively high precision control, low health hazards, less processing time and appreciable versatility in application at comparatively low cost over other modification process as stated above [12,13]. The TIG arcing process have the capability to modify the surface in greater depth (ranging in mm) as compared to carburizing and nitriding [12]. Such superior depth of modification is made possible by applying instant intense heat by arcing over the surface, which results a quick localised surface melting. It establishes a large thermal gradient due to relatively strong effective heat sink around the fused zone similar to laser and electron beam surfacing [14]. Here a critical control of thermal gradient gives desired microstructure by dictating phase transformation in the fusion zone (FZ). Further the size and shape of the fused zone (FZ) are controlled by heat input (Ω) under combinations of TIG parameters (current, voltage and travel speed) where, argon gas used as shielding. A through study on microstructure and hardness behaviour of the fused zone (FZ) and heat affected zone (HAZ) of surface modified AISI 4340 steel conducted by sudhir et. al. [15]. Consequently the effect of TIG arcing on tensile and tensile-fatigue properties is critically examined and discussed in a previous article [16]. It is observed that the TIG arcing significantly improves the surface hardness in FZ up to an appreciable depth at optimum process parameter. It is also understood that the TIG arcing may introduce residual stresses on the modified surface which has appreciable influence on its mechanical properties, that should be more thoroughly understood for mechanical characterization of the matrix. In consideration of the above observations, an effort was done to understand thoroughly the effect of TIG arcing behaviour on AISI 4340 steel substrate. The “three dimensional double ellipsoidal heat source model” proposed by Nguyen et al. (1999) [17] is used to validate the experimental work in the context of the understanding on the bead geometry and its thermal cycle. Effect of thermal behaviour of the modified matrix, affecting its microstructure and residual stress generation, on the uniaxial bending properties of the surface modified substrate under static and dynamic loading has been studied. 2.
ANALYTICAL ESTIMATION OF ARC BEHAVIOUR The estimation given by Nguyen et al. (1999) [17] for the analysis of a double
ellipsoidal moving point heat source (Fig. 1) of arc welding. In view of that this model has been used for analytical estimation of isotherms and thermal cycle produced by controlled
3
fusion using TIG arcing process [14]. The analytical approach followed in this exercise has been stated below.
Fig.1 Graphical representation of analytical model of heat source [14]. At a location (x,y,z) in semi-infinite plate, the temperature Td can be estimated as follows [14].
dt' . ' 2 ' 2 1 2 a (t t ) + a . 1 2 a (t t ) + b h h 3 3 .Q A W t Td = ∫ 2 ρ .c .π π 0 A' B' + 1 2 a (t - t ' ) + c h2 f 1 2 a (t - t ' ) + c h2 b
(
) (
(
)
)
(
)
+ T .......(1) 0
Where, QAW is the arc heat transferred to the weld pool and estimated by equation 2. QAW = η.V .I Joule .........................(2) 3(x - v.t ' ) 2 3y 2 3z 2 A ' = rf .exp ......................(3) ' 2 ' 2 ' 2 12a(t - t ) + c hf 12a(t - t ) + a h 12a(t - t ) + b h 3(x - v.t ' ) 2 3y 2 3z 2 B' = rb .exp ..........................(4) ' 2 ' 2 ' 2 12a(t - t ) + c hb 12a(t - t ) + a h 12a(t - t ) + bh
Where, rf (front) and rb (behind) are the proportion coefficients of heat source, calculated as rf =
2.c hf
( c hf
+ c hb )
...........................(5)
4
rb =
2.c hb
and
( c hf
+ c hb )
.............................(6)
-
η is arc efficiency considered as 75%,
-
Arc voltage is denoted by V and arcing current is denoted by I.
-
The material constants mass density(ρ) , specific heat(c) and thermal diffusivity (a).
-
ah, bh, chf and chb are the ellipsoidal parameters as shown in fig. 1. The heat source parameters chf and chb are knowing as chf = ah and chb = 2 chf [18].
3.
EXPERIMENTAL
3.1
Surface modification AISI 4340 steel plate having dimension of 150×75×10 mm in length, width and
thickness respectively was used in this investigation. Chemical wt% in medium carbon AISI 4340 steel is shown in Table-1. It is also known as Ni-Cr-Mo steel on the basis of its alloying content. Prior to treatment the substrate surface was cleaned by using surface grinding followed by acetone cleaning. The process was carried out by non-consumable tungsten electrode having 3.2 mm diameter. Argon as a shielding gas (15 l/min) was used for TIG arcing process. The primary parameters (I and S) are affecting the surface modification by their significant influence on Ω of TIG arcing process, they are varied I from 80 to 140 A and S from 6 to 15 cm/min and studied their influence on characteristics of the modified matrix. Ω of the TIG arcing process was estimated as follows[19]. Ω = [(η×I×V×60)/(S×1000)]×η
------------- (6)
Where, Ω is heat input (kJ/mm) -
I considered as arcing current
-
S considered as arc travel speed (mm/min)
-
η considered as arc efficiency (0.75)
-
V considered as arc voltage (V= 10.5±1.0 V).
The temperature and thermal cycle of at a desired locations corresponding to the FZ and HAZ has been measured by R- type thermocouple.
5
Table 1. Chemical composition of AISI 4340 steel. C
Cr
Ni
Mo
Mn
S
P
Si
Fe
0.39
0.79
1.65
0.22
0.71
0.04
0.04
0.21
remaining
3.2
Microstructure Microstructure of the substrate were studied on their transverse section by preparing
them using standard metallographic procedure followed by etching in alcoholic 5% nitric acid solution (nitric acid 5% and remaining part is ethanol). Macrograph of the profile of the fused modified region and its microstructure was studied under optical microscope. The profile of the FZ and HAZ was measured under optical macroscope by placing the specimen on a sliding table capable to move in the direction of X and Y coordinate with the help of in built screw gauge micrometre.
3.3
Mechanical testing Hardness measurement of modified substrate was carried out on its metallographically
prepared and etched transverse section using Vickers micro-hardness tester at a load of 300 grams with 30 s dwell time A 0.5 mm distance from its top surface was taken for hardness measurement in FZ and for the same of the HAZ the indentation was made in the matrix at 0.3 mm from the fusion line. The hardness of surface modified substrate was studied at the low and high Ω of 0.3 to 0.6 kJ/mm respectively. Similarly hardness of the base metal was also studied and compared. Mechanical tests were performed on INSTRON Servo hydraulic machine with load cell limit of ± 50 kN and ±75 mm cross head movement. Tests were performed at room temperature. Schematic diagram of the specimen and the loading system has been shown in Fig. 2, where modified zone experiences maximum tensile stress with load applied at the centre. The sample size of both the tests was kept as 150×W×9 mm for length, width and thickness respectively where, W varied with modified bead width. ASTM E-855-08 “Standard Test Method for Bend Testing of Metallic Flat Materials” [20] was used with a strain rate of 1.0 mm/min. Average result of three samples were reported. Estimation of stresses under the three-point bending load was also done according to ASTM E-855-08. Flexural stresses (σ) in MPa were calculated by eq. 7, in which, M- bending moment (N.mm),
6
Y- perpendicular distance from neutral axis (mm) and Z considered as moment of inertia (mm4). σ = M*Y/Z
---------- (7)
M and I are estimated as M = F*L/4
--------- (8)
I = W*T3/12
---------- (9)
Where, -
F is force applied (N).
-
L, W, and T are sample dimension as shown in fig. 2. The fatigue test was done at 10 Hz frequency. Stress ratio (R) is taken as 0.1. where R
is the ratio of minimum (σmin)and maximum (σmax) applied stresses, respectively. The endurance limit was considered as 1×106 cycle after that the test was stop by considering no failure in the sample. Representation of fatigue test results was compliance with ASTM E739 “Standard Practice for Statistical Analysis of Linearized Stress-Life (S-N) Fatigue Data” [21]. In case of failure the fracture surface was studied under scanning electron microscope.
Fig. 2 Static and dynamic three point bend test sample for Single-pass surface modification. 3.4
Residual stress analysis 7
ASTM E-837 “Standard Test Method for Determining Residual Stresses by the HoleDrilling Strain Gage Method” [22] was used. A strain gauge rosette (FRS-2-17) [23] was used. In a strain gauge rosette three strain gauges of the rosette, located at 00, 900 and 2250 in radial direction as shown in Fig. 3, are used to find out the strain and stresses in different orientations. The residual stresses in different direction were estimated by using eqs. given below. Longitudinal direction = P – Q
-------
Transverse direction = P + Q
-------
(10) (11)
Where, -------
1
-------
(13)
-
ε1, ε2 and ε3 are the measured strain.
-
E is Modulus of Elasticity (210 GPa) and ν is Poisson’s ratio (0.29) respectively considered as material constants.
-
ā and
are calibration coefficients [16].
Fig. 3 A systematic view of residual stress measurement setup [16].
4.
(12)
RESULTS AND DISCUSSIONS
8
4.1.
Geometry of modified zones Typical macroscopic cross sectional view of the TIG modified surface by changing I
from 80 to 140 A at constant V and S of 10.5±1.0 V and 9 cm/min respectively as shown in Fig. 4. The figure shows that the area of fusion modified zone varies significantly with the change in process parameters affecting the Ω.
Fig. 4 Typical macroscopic cross sectional view of modified region. The geometry of the modified zone has been characterized by four different factors such as 1. Depth of penetration (Pd), 2. Bead width (Wb), 3. Width of HAZ (Whaz) and 4. Area of FZ (AFZ). At the given V of 10.5±1.0 V the effect of variation in I and S on the WB, Pd, Whaz and AFZ has been shown in Figs. 5 (a-d) respectively. The figures show that the increase in I from 80 to 160A significantly increases the Wb, Pd, Whaz and AFZ. It has been further observed that the WB, Pd, Whaz and AFZ increase appreciably with the reduction of S. Such a variation in WB, Pd, Whaz and AFZ as a function of I and S may be primarily attributed to the variation in Ω to FZ governing the size and fluidity of molten pool [24]. The empirical correlations of the WB, Pd, Whaz and AFZ as a function of I and S depicted in Figs. 5 (a-d) respectively have been given below.
W B = − 0.12213 S + 0.04532 I + 9.17 × 10 −5 SI + 0.5204
……
(14)
Pd = − 0.0172 S + 0.02617 I − 5.48 × 10 −4 SI − 0.2314
……
(15)
Whaz = −0.71S + 1.04 ×10−2 I + 7.33 ×10−6 IS + 0.956
……
(16)
AFZ = 0.496S + 0.252I − 1.09 ×10−2 IS − 13.11
……
(17)
9
Fig. 5 Effect of I and S on (a) Pd, (b) Wb (c) Whaz and (d) AFZ at an V of 10.5 ± 1.0 V. 4.2 Thermal characteristics of the modified zone Thermal characteristics of the fusion zone is critically controlled by the weld pool due to their influence on contact area with the conductive substrate and exposed area for other types of heat dissipation as well as heat content of fused mass. Thus, it affects solidification mechanism of FZ and consequently influences the microstructure and properties of the solidified matrix. The shape and size of FZ and HAZ controlled by the Ω through variation of I, V and S may be clearly understood from the isotherm of fused zone of the substrate. The isothermal curves are estimated by the heat flow expression (eq. 1) as discussed earlier. The size of FZ and HAZ is illustrated by the isotherm at different Ω as a function of I, V and S. The isothermal curves in both the planes of x-y and y-z, thermal cycle and cooling rate are estimated by the expression given in chapter 2 equation 1. 4.2.1 Isothermal curves in different planes
10
At a given V of 10.5±1.0 V the effect of variation in I (80 to 140 A) and S (6 to 15 cm/min), on FZ isotherm in XY plane on substrate surface has been shown in Figs. 6 (a-b). Similarly, FZ isotherm in YZ plane on transverse to the direction of arc travel is shown in Figs. 7 (a-b). The Fig. 6 (a) depicts that the increase of I increases the width and length of isotherm at a given S, whereas at a given arc current the increase in S significantly decreases the width and length of the isotherm as shown in Fig. 6 (b). Similarly the Fig. 7 (a-b) depicts that the size of isotherm of modified zone increases with increase of I. This is primarily attributed to the increment in heat transfer to the FZ with the increase of I at a given S. The figures also reveal that the increase of S at a given I and V reduces the width, length and depth of the isotherm. This is primarily happened at decrease in heat transfer per unit length by increase in speed of arc travel. The increment in S at a given I reduces the energy input per unit length, which influences the distribution of energy into the work piece. The increment in S also reduces the extent of distribution of energy into the substrate. The Fig. 7 (a-b) further depicts a comparison of isotherm of the modified zone at I (80 to 140 A) and S (6 to 15 cm/min). The geometry of modified zones of substrate, defined as the fusion zone and HAZ in concurrence to their estimated zone of peak temperatures of 1700 K and 1073 K respectively, are measured as a signature of relevant change of microstructure in the matrix. The figure shows that the predicted geometry (broken line) of modified zone is quite close to their experimental observations (continuous line) with a difference of 4 ± 2 %.
11
Fig. 6 The estimated isotherms in XY plane at given V 10.5±1.0 V of (a) varied I of 80, 100, 120 and 140 A and (b) varied S to 6, 9, 12 and 15 cm/min.
12
Fig. 7 Comparison of estimated and measured isotherms in YZ plane at given V 10.5±1.0 V Effect of (a) varied I to 80, 100, 120 and 140 A and (b) varied S to 6, 9, 12 and 15 cm/min.
13
4.2.2
Thermal Cycle Thermal cycle shows the nature of variation in temperature during heating and
cooling with respect to time in FZ and HAZ. At a given range of Ω of 0.3 – 0.60 kJ/mm, the typical thermal cycle of FZ and HAZ developed at I as 80 and 140 A has been shown in Figs. 8 (a-b) while the V and S were kept constant as 10.5±1.0 V and 12 cm/min. The figure reveals a close approximation of the measured and estimated thermal cycle at different depth of Z axis (Fig. 1) relevant to the FZ and HAZ of different arcing process parameters. It also depicts that the increase of Ω by increase in I reduces the cooling rate (CR). Such changes were occurred due to reduction in area of effective heat sink of the substrate and lower temperature gradient for heat transfer from the relatively larger fusion zone and wider HAZ at higher Ω. This has caused a reduction in cooling rate from 800 to 500◦C [25,26].
Fig. 8 Thermal cycle at different depth of Z axis correspond to the FZ and HAZ at the I and Ω of (a) 80 A; 0.30 kJ/mm respectively and (b) 140 A; 0.60 kJ/cm respectively.
14
Fig. 9 FZ and HAZ cooling rate in inter-critical temperature range (800-5000C). A comparison of measured cooling rate and estimated cooling rate at the inter-critical temperature range (800-5000C) of FZ and HAZ obtained from respective thermal cycle plots of different I and Ω has shown in fig. 9. The figure reveals that as Ω increase by increase of I leads to decrease in cooling rate in FZ as well as HAZ. It was also noticed that the estimated cooling rate gives a close approximation with measured cooling rate. The final phase transformation of the FZ and HAZ was primarily governed with the changes in isotherm and cooling rate . 4.3
Microstructure Proeutectoid ferrite and pearlite were observed in microstructure of the substrate as
shown in Fig. 10 and having hardness of 256 ± 10 HV. Typical microstructures of the fusion modified zone prepared at relatively low (0.3 kJ/mm) and high (0.6 kJ/mm) Ω of varying arc current to 80 and 140 A employed at a given S and V of 12 cm/min and 10.5±1.0 V respectively has been shown in Fig. 11(a(i) and b(i)) respectively. The micrograph presented in Fig. 11 (a(i)) reveals the significant transformation of needle shape martensite beside acicular ferrite. However, as the Ω is increased from 0.30 to 0.60 kJ/mm, the transformation is predominated by the formation of lath martensite in the matrix containing acicular and patches of ferrite as shown in Fig. 11 (b(i)). Such transformation was critically attributed to cooling rate, which is controlled by the arcing parameter as discussed above. Similarly, phase transformation observed in the HAZ of base material at Ω 0.3 and 0.6 kJ/mm has been shown in the microstructures presented in Figs. 11 (a(ii)) and (b(ii)) respectively. The Fig. 11
15
a(ii) reveals that martensite along with bainite and allotriomorphic Ferrite in the matrix was seen at the Ω of 0.3 kJ/mm, as the Ω increase to 0.6 kJ/mm, the martensitic transformation was supressed and relative amount of bainite and pearlite transformation in the matrix. It was also noticed that the grain coarsening were observed as the Ω increase from 0.30 to 0.60 kJ/mm shown in figs. 11 a(ii) and b(ii).
Fig. 10 Microstructure of the AISI 4340 steel substrate
Fig. 11 Microstructures of (i) FZ and (ii) HAZ under varied Ω of different I a) 0.3 kJ/mm, I= 80 A and b) 0.6 kJ/mm, I= 140 A in modified surface at constant S and V of 12 cm/min and 10.5 ± 1.0 V respectively.
16
The significant increase of hardness of the substrate from 256±10 HV confirms the observed change in microstructure in its FZ and HAZ with a change of Ω from 0.3 to 0.6 kJ/mm as discussed above. With increase of Ω relatively low to high, the hardness of the FZ of the medium carbon substrate reduces from 745±8 HV to 650±16 HV, while HAZ hardness reduces from 567±12 HV to 435±22 HV. Such changes in hardness of FZ and HAZ related to Ω, that may have primarily caused by microstructural transformation from fine niddle shape martensite to lath martensite in the fusion zone and relatively coarser martensite to bainite in HAZ respectively due to reduction in cooling rate as stated above. 4.3 Flexural properties The flexural yield strength and maximum strength of as received AISI 4340 structural steel under bending load (Fig. 2) are found as about 540 and 1160 MPa respectively. At different S of 6, 9, 12 and 15 cm/min, the variation in flexural properties of surface modified substrate prepared at Ω in the range of 0.24 to 1.21 kJ/mm by changing the I has been shown Figs. 12 (a-d) respectively. It is observed that the single-pass TIG arc surfacing significantly affects the bend properties such as flexural properties of a component as shown in Figs. 13 (a-b). The figures reveal that a low Ω below 0.4 kJ/mm often shows a tendency to maximum lowering of the flexural properties as compared to base metal. This has happened due to increase of cooling rate at low Ω of weld pool, promoting hard phase transformation in it. This is found as formation of needle shape martensite in the matrix, which has significantly low ductility that lead to sudden failure. Formation of residual stress due to such hard phase transformation may have also put an adverse effect on flexural yield strength. Further, it is observed that as the Ω is increased by decreasing S from 12 cm/min to 6 cm/min, there is an appreciable increase in flexural yield strength as well as flexural maximum stress and flexural extension as shown in Fig. 12 (a-c). Such behaviour has happened primarily due to relatively softer lath martensite transformation in the matrix at relatively slower cooling rate of higher Ω (Fig. 9). The Fig. 13 (a) shows that at the S of 6 cm/min, there is maximum improvement in flexural yield strength of the order of 25 to 35 % with respect to base metal observed at approximately similar maximum flexural stress. Whereas, maximum reduction in flexural properties is observed at an I and S of 80 A and 15 cm/min respectively.
17
Fig. 12 Flexural stress-extension plots by different Ω by changing I at different S of (a) 6, (b) 9, (c) 12 and (d) 15 cm/min. Typical fracture surface of the specimen failed under three point bend test has been shown in Fig. 14. The fracture surface reveals the occurrence of brittle fracture in the modified zone followed by ductile fracture in base material primarily due to presence of relatively brittle phase in the fusion modified matrix. Micro cracks were observed at the centre of the modified zone, which may have generated by the presence of high stresses that may cause failure in the plane parallel to that of the applied load.
18
Fig. 13 An effect of different I and S on a) Flexural yield strength, b) flexural maximum strength, observed during three point bend test.
Fig. 14 Macro and micro fractographs of samples failed under bending load of flexural test. 4.4
Fatigue properties under bending load
19
Fatigue life of the base material and modified surface under bending load has shown in Figs. 15 (a-d). The figure reveals the fatigue life of the modified surface has been significantly decreased as compare to base material. It is observed that at the Ω in the limit of 0.24 to 0.32 kJ/mm the endurance limit decreases from σmax of 480 to 405±5 MPa with respect to the base metal. However, Fig. 15 (a-d) also depicts that with the increase of Ω from 0.24 to 1.21 kJ/mm, an improvement in σmax from about 400 to 500 MPa at the endurance limit of modified substrate was observed. Such improvement is in agreement to the flexural strength and microstructural changes under the surface modification as previously discussed. At a constant V and S of 10.5±1.0 V and 6 cm/min respectively, an approximately same endurance limit with respect to the base material (σmax of 480 MPa) is observed when the Ω lies in the range of 0.99 - 1.21 kJ/mm. But there is a reduction in finite fatigue life as shown in Fig. 15 (a). Hard phase transformation in the matrix is attributed to change in slope of S-N curve, which is relatively reduced from that of the base metal.
Fig. 15 The behaviour of surface modified steel S-N curve by changing I and S of (a) 6, (b) 9, (c) 12 and (d) 15 cm/min.
4.5 Residual stresses
20
Residual stresses are formed due to differential thermal gradient and microstructure modification of the substrate during arcing in FZ [27–29]. At different depth of the substrate in reference to its modified surface, the variation of residual stresses with respect to direction of the weld bead has shown in Figs. 16 (a and b). The figures show that increase in Ω enhances the residual stresses at every location of the matrix of the modified substrate. However, the Fig. 16 (a) depicts that the increase of Ω from 0.24 to 1.21 kJ/mm appreciably enhances the longitudinal residual stress in the matrix close to top surface of the substrate to about 100 and 160 MPa respectively followed by a significant reduction to compressive stress of about −225 and −25 MPa respectively at depth of about 0.4 mm from the surface. Here it is interestingly noted that the reduction of residual stress is significantly higher in case of the lower Ω with respect to that marked at higher Ω. The Fig. 16 (b) shows that the transverse residual stresses in the matrix close to the top modified surface of the substrate increases from about 210 to 315 MPa with the increase of Ω from 0.24 to 1.21 kJ/mm. But magnitude of stresses continuously reduces significantly with the increase of depth from the surface. Here it is interestingly observed that up to a depth of about 0.2 mm the residual stresses present in the matrix are of tensile nature whereas at a larger depth beyond about 0.25 mm the residual stresses show a tendency to be of compressive nature, especially at the lower Ω of 0.24 kJ/mm. The presence of tensile stresses in both directions at top surface of the modified zone may have a significant adverse effect on fatigue properties during application of unidirectional bending load (Fig. 2). It may have primarily happened due to additional support of residual stresses to the applied stress for enhancement of resolved stress [30].
21
Fig. 16 At a given arc parameter a) Variation in longitudinal stresses b) Variation transverse stresses. The resolved stresses for three point flexural bend stresses were calculated by the eq. 18. i =5
σ resolved = ∑ σ a + σ r i =−5
i
i
……………………………..……. (18)
Where σa is applied stress due to load, σr is residual stresses due to surface modification and i is thickness of the flexural sample (about 10 mm thickness). Here i=0 mm means neutral axis of the sample. The modified surface resolved stress at lower fibre under the flexural test of single-pass TIG arc modified sample prepared at the Ω of 0.24 to 1.21 kJ/mm is shown in Fig. 17. The resolved stress was reached to about 725 MPa up to a depth of 0.15 mm followed by a sudden reduction to 370 MPa, when the applied stress of testing was 600 MPa. During flexural test, availability of tensile residual stresses play a relatively more prominent role for crack initiation and propagation that adversely affects the flexural properties in static as well as dynamic loading.
Fig. 17 Effect of residual stresses on resolved stress under bending load. 22
5. Conclusions •
Thermal characteristics of TIG arcing such as isotherm, thermal cycle and cooling rate of AISI 4340 steel has been established with the help of an analytical model and experimental measurement.
•
Analytical results are found closely in agreement of the order of 94% to the measured values. The three point bend properties of the substrate under the static and dynamic loading are significantly affected by TIG arc surface modification, which is primarily attributed to development of residual stresses and its microstructural modification.
•
Surface modification has improved the flexural properties of the substrate with a maximum improvement of flexural yield strength of the order of 25 to 35 % during processing at relatively slow S of 6 cm/min, while the Ω lies in the range of 0.6-1.21 kJ/mm. -
This has primarily happened because the increase of cooling rate at higher S promotes transformation of fine niddle shape martensite in the fusion modified matrix that significantly enhances its hardness to 745±8 HV with a decrease in ductility and consequently reduces the bending properties under the static and dynamic loading.
•
The increase of Ω from 0.24 to 1.21 kJ/mm improves σmax from about 400 to 500 MPa at the endurance limit of fatigue life of the surface modified substrate. Further development of more residual stresses of tensile nature at the surface modified at higher Ω adversely affect the mechanical properties under both the static and dynamic bending load due to enhancement of resolved stresses in it by additional support to the applied stress.
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Date- 02, October 2019 SUBJECT: Declarations of interest
Dear Editor I am enclosing herewith a manuscript entitled “Thermal behaviour of TIG arc surfacing affecting mechanical properties of AISI 4340 steel substrate under static and dynamic loading” for publication in “Materials Science and Engineering: A” for possible evaluation. I also like to state that the “Declarations of interest: none” for this research article.
Sudhir Kumar