Failure Mechanism of Gun Barrel Caused by Peeling of Cr Layer and Softening of Bore Matrix

Article Failure Mechanism of Gun Barrel Caused by Peeling of Cr Layer and Softening of Bore Matrix

Zi-meng Wang 1, Chun-dong Hu 1,*, Yang-xin Wang 1, Heng-chang Lu 1,*, Jun-song Li 2 and Han Dong 1

1 School of Materials Science and Engineering, Shanghai University, Shanghai 200444, China; [email protected] (Z.-m.W.); ﬂ[email protected] (Y.-x.W.); [email protected] (H.D.) 2 No. 208 Institute of China Ordnance Industry, Beijing 102202, China; [email protected] * Correspondence: [email protected] or [email protected] (C.-d.H.); [email protected] (H.-c.L.); Tel.: +86-131-6209-5128 (C.-d.H.)

Abstract: Research on failure mechanism is essential for the prolonging of gun barrel lifetime. To explore the gun barrel failure mechanism, the damage characteristics of a machine gun barrel were characterized. The results show that the failure of the gun barrel is correlated with the peeling of the Cr layer on the bore surface and the softening of the bore matrix. The damage of the Cr layer varies along the axial direction. From the gun tail to the muzzle, the damage mode of the Cr layer changes from peeling to wearing. The damage rate in both the tail and the muzzle is higher than that in the middle of the barrel. The matrix close to the bore surface is softened due to the high temperatures caused by stress–relief–annealing and shooting. The gun tail suffers from higher temperatures, thus being softened more seriously than the other parts. The softened matrix results in an increase in the tendency of plastic deformation of the bore surface and an increment in the bore diameter, which leads to a decrease in the firing accuracy. Keywords: gun barrel; failure mechanism; crack propagation; bore matrix softening; 3D intelligent Citation: Wang, Z.-m.; Hu, C.-d.; hyperfield microscopy Wang, Y.-x.; Lu, H.-c.; Li, J.-s.; Dong, H. Failure Mechanism of Gun Barrel Caused by Peeling of Cr Layer and Softening of Bore Matrix. Metals 2021, 1. Introduction 11, 348. https://doi.org/10.3390/ It has been widely acknowledged that the failure of a gun barrel is correlated to met11020348 thermal, chemical, or mechanical actions. The enlargement of bore diameter could lead to a reduction in both muzzle velocity, range and a decrement in firing accuracy [1–5]. Academic Editor: Filippo Berto The research on the failure mechanism of the gun barrel has been receiving increasing Received: 20 January 2021 Accepted: 16 February 2021 interest. Cote et al. [6] proposed a model describing the mechanism of gas–solid erosion, Published: 19 February 2021 which indicates that the erosion is mainly caused by the reaction between gunpowder exhaust and matrix under high temperature and pressure conditions. Lawton et al. [7]

Publisher’s Note: MDPI stays neutral correlated the wear of bore surface with initial temperature, maximum surface temperature, with regard to jurisdictional claims in and gunpowder composition. Their study indicates the importance of heat and gunpowder published maps and institutional affil- composition on erosion. Sopok et al. [8–10] developed a thermo–chemical–mechanical ero- iations. sion model, defining different erosion areas, and annotating different erosion mechanisms. Underwood et al. [11–14] analyzed gun barrel damage through a thermodynamic model and a fracture model. Qiao et al. [15] investigated the bore damages in a failed large-caliber machine gun barrel and found that the bore surface in the tail region was largely damaged by erosion and that the damage in the four and five cones is dominant for the failure of the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. gun barrel. This article is an open access article The bore damage caused by erosion and/or wear during shooting is a vital factor distributed under the terms and influencing the barrel life. According to the forementioned mechanisms of erosion and conditions of the Creative Commons wear, the damage degree and mode of the Cr layer on the bore surface vary along the axial Attribution (CC BY) license (https:// direction. In our previous studies [16,17], the Cr layer on the groove lines in the tail region creativecommons.org/licenses/by/ was observed to be damaged prior to that on the land lines. The damage of the Cr layer 4.0/).

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layer aggravates the damage of the matrix and increases the tendency toward enlarging aggravates the damage of the matrix and increases the tendency toward enlarging the bore the bore diameter. diameter. In this paper, 3D intelligent hyperfield microscopy (3DIHM) was for the first time In this paper, 3D intelligent hyperﬁeld microscopy (3DIHM) was for the ﬁrst time introduced into the study of gun barrel damage. With 3DIHM, the damage characteristics introduced into the study of gun barrel damage. With 3DIHM, the damage characteristics of a machine gun barrel were characterized and the corresponding failure mechanisms of a machine gun barrel were characterized and the corresponding failure mechanisms discussed. discussed.

2.2. Materials Materials and and Methods Methods TheThe material material used used in in this this study study was was a a Cr–N Cr–Ni–Mo–Vi–Mo–V steel steel with with chemical chemical compositions compositions (in(in wt.%): wt.%): 0.31 0.31 ±± 0.0080.008 C, C, 0.25 0.25 ± 0.01± 0.01 Si, Si,0.24 0.24 ± 0.01± 0.01 Mn, Mn, 0.009 0.009 ± 0.001± 0.001 P, 0.004 P, 0.004 ± 0.001± 0.001 S, 1.92 S, ±1.92 0.01± Cr,0.01 1.42 Cr, ± 1.42 0.009± Mo,0.009 and Mo, 0.31 and ± 0.310.01 ±V.0.01 Each V. firing Each ﬁringcycle was cycle carried was carried out at outthree at temperatures:three temperatures: room roomtemperature, temperature, high temp high temperatureerature (50 ± (502 °C)± 2and◦C) low and temperature low temperature (−49 ±(− 2 49°C).± The2 ◦C bore). The diameter bore diameter was measured was measured with a with ruler a gauge, ruler gauge, for which for which the measurement the measure- methodment method can be can found be found in the in literature the literature [15]. [When15]. When the projectile the projectile volume volume was was 0, 7, 0, 24, 7, 24,..., or ..., 100%or 100% of the of the full full life, life, they they were were marked marked as as 0% 0% life, life, 7% 7% life, life, 24% 24% life, life, ..., ..., or or 100% 100% life. The criteriacriteria for for the the test test of of gun gun barrel barrel life life were were as as follows: follows: (i) (i) the the reduction reduction rate rate of of initial initial muzzle muzzle velocityvelocity was was greater greater than than or or equal equal to to 15%; (i (ii)i) the number of elliptical bullet holes (the ratioratio of of major major axis axis to to minor minor axis axis is is greater greater th thanan 1.25) 1.25) which which exceeded exceeded 50% 50% of of the the projectile projectile number;number; (iii) (iii) the the average average dispersion dens densityity of three consecutive targets was R50R50 ≥ 3030 cm; cm; andand (iv) (iv) visible visible cracks cracks appeared. appeared. Provided Provided th thatat one one of of the the criteria criteria was was reached, the gun barrelbarrel was was declared declared to to be failed. TheThe barrel barrel used used in in this this study study was was a a large- large-calibercaliber machine machine gun gun barrel. barrel. To To explore the damagedamage characteristics characteristics of of the the gun gun barrel, barrel, se sevenven samples samples were were taken with the positions beingbeing 113–123 mm,mm, 133–233133–233 mm, mm, 243–253 243–253 mm, mm, 343–353 343–353 mm, mm, 543–553 543–553 mm, mm, and and 743–753 743–753 mm, mm,respectively respectively (the tail(the as tail the as starting the starting point). point). Thesamples The samples were namedwere named MG1, MG1, MG2, MG2, MG3, MG3,MG4, MG4, MG5, MG5, MG6, MG6, and MG7, and MG7, correspondingly correspondingly (see Figure (see Figure1). It should1). It should be noted be noted that only that onlyMG2 MG2 was awas longitudinal a longitudinal sample sample and theand rest the wererest were ring samples.ring samples.

Figure 1. Sample positions in a large-caliber machine gun barrel. Figure 1. Sample positions in a large-caliber machine gun barrel.

AllAll samples samples were were first ﬁrst cleaned cleaned by by ultrasonic wave in an acetone solution, and then thethe ring ring samples samples were were ground ground and and polished polished after after being being inlaid inlaid in in epoxy epoxy resins. resins. The The cross- cross- sectionalsectional morphologies morphologies of of the the samples samples were were observed observed with with Smart Smart Zoom Zoom 5 5 3DIHM 3DIHM manu- manu- facturedfactured by by Carl Carl Zeiss Zeiss company company (Oberkoche (Oberkochen,n, Germany). Germany). A A Nova Nova NanoSEM NanoSEM 450 450 scanning scanning electronelectron microscope microscope (SEM) (SEM) pr producedoduced by by FEI FEI company company (Hillsboro, (Hillsboro, OR, OR, USA) USA) was was used used for for thethe characterization characterization of of the the microstructures microstructures of of the the samples. All All samples samples for for the the SEM SEM test werewere corroded corroded with 4% nitric acid alcohol alcohol solution. Both Both alcohol alcohol and and nitric nitric acid acid were were made made byby China NationalNational PharmaceuticalPharmaceutical Group Group Corporation Corporation (Beijing, (Beijing, China). China). Then Then 4% nitric4% nitric acid acidalcohol alcohol solution solution was was prepared prepared in proportion. in proportion. A Wilson A Wilson hardness hardness tester tester was was used used for thefor themeasurement measurement of radial of radial Vickers Vickers hardness, hardness, with with a load a load of 1000 of 1000 g. The g. hardnessThe hardness of MG1, of MG1, MG5 MG5and MG7 and MG7 in eight in eight directions directions (not including(not including the Cr the layer) Cr layer) was tested. was tested.

3.3. Results Results 3.1.3.1. Bore Bore Diameter Diameter Variation Variation along along Axial Axial Direction Direction TheThe variation variation of of bore bore diameter diameter of of the the gun gun barrel, spanning 0–100% 0–100% life, life, is is shown in FigureFigure 22.. For the convenience of depiction, thethe whole barrel was divided into three parts: thethe tail, tail, middle, middle, and and muzzle. muzzle. According According to to this this division, division, MG1 MG1 and and MG2 MG2 are are in in the the tail, tail, MG3, MG4, MG5, and MG6 are in the middle, and MG7 is in the muzzle. From the tail to MG3, MG4, MG5, and MG6 are in the middle, and MG7 is in the muzzle. From the tail to the muzzle, the bore diameter decreases ﬁrst and then increases. With the increasing gun

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the muzzle, the bore diameter decreases first and then increases. With the increasing gun life,life, the the increment increment in in the the bore bore diameter diameter in in both both the the tail tail and and muzzle muzzle regions regions is is larger larger than than thatthat in in the the middle middle part. part.

13.05 MG1 MG2 MG3 MG4 MG5 MG6 MG7 100% 91% 71% 13.00 12.95 12.90 60% 12.85 39% 12.80 24% 12.75

Bore diameter/mm 12.70 7% 12.65 0% 12.60 0 200 400 600 800 1000 1200 Axial distance/mm FigureFigure 2. 2.Variation Variation of of bore bore diameter diameter of of a a gun gun barrel barrel during during the the 0–100% 0–100% life life span. span.

3.2.3.2. Bore Bore Surface Surface Damage Damage TheThe cross-sectional cross-sectional morphologies morphologies of of MG1, MG1, MG3, MG3, MG5, MG5, and and MG7 MG7 taken taken by by 3DIHM 3DIHM are are shownshown in in Figure Figure3. 3. The The red red circle circle in in the the pictures pictures represents represents the the initial initial bore bore diameter, diameter, which which isis 12.7 12.7 mm. mm. Before Before shooting, shooting, the the whole whole bore bore surface surface is is covered covered with with a a Cr Cr layer. layer.Figure Figure3 a3a showsshows that that MG1 MG1 (in (in the the tail) tail) presents presents a state a ofstat seriouse of serious damage, damage, where crackswhere extendcracks deeply extend intodeeply the matrixinto the from matrix the borefrom edge the bore with edge the long with cracks the long being cracks uniformly being distributed. uniformly Nodistrib- Cr layeruted. remains, No Cr layer and theremains, bore diameter and the isbore obviously diameter expanded. is obviously As shownexpanded. in Figure As shown3b, the in damageFigure 3b, degree the damage of MG3 (indegree the middle) of MG3 is(in significantly the middle) reduced. is significantly The Cr reduced. layer on theThe grooves Cr layer fallson the largely grooves off. falls The largely cracks off. in the The exposed cracks in matrix the exposed are short matrix and are dense. short The and maximum dense. The thicknessmaximum of thickness the Cr layer of the on theCr layer lands on of the MG3 lands is about of MG3 190 is± about4 µm. 190 The ± damage4 μm. The degree damage of MG5degree is furtherof MG5 reduced is further (see reduced Figure (see3c). Figure Only a 3c small). Only part a ofsmall the part Cr layer of the on Cr the layer grooves on the fallsgrooves off and falls only off a and small only quantity a small of shortquantity cracks of short is observed. cracks Theis observed. maximum The thickness maximum of ± µ thethickness Cr layer of on the the Cr lands layer ofon MG5 the lands is about of MG5 100 is about4 m. 100 In MG7 ± 4 μ (Figurem. In MG73d), (Figure the lands 3d), are the largelylands are worn largely off, leaving worn off, the remainderleaving the being remainder aligned being to the aligned Cr layer to onthe theCr grooves.layer on Inthe MG7,grooves. the CrIn layerMG7, isthe only Cr observedlayer is only on the observ groovesed on and the almost grooves no and cracks almost are present. no cracks are To characterize the degree of bore surface damage, a rifling retention X is defined as present. To characterize the degree of bore(d surface− 12.7 damage,) a rifling retention X is defined as S = min × 100% (1) min (d12.7− 12.7) S = min × 100% (1) min 12.7 (d max − 12.7) Smax = (d − 12.7)× 100% (2) S = 12.7max × 100% (2) max 12.7 Smax X = S (3) X S=min max (3) S where S is the expansion rate of bore diameter andmind is the bore diameter. The subscripts “min”where and S is “max” the expansion refer to therate minimum of bore diameter and maximum and d is states,the bore respectively. diameter. The The subscripts value of X“min”reflects and the “max” degree refer of rifling to the retention, minimum where and maximum a larger value states, indicates respectively. a better The degree value of of riflingX reflects retention. the degree of rifling retention, where a larger value indicates a better degree of rifling retention.

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Figure 3. Cross-sectional morphologies of the samples taken by 3D intelligent hyperﬁeld microscopy Figure 3. Cross-sectional morphologies of the samples taken by 3D intelligent hyperfield micros- (3DIHM):copy (3DIHM): (a) MG1; (a (b) MG1;) MG3; (b (c) )MG3; MG5; (c and) MG5; (d) MG7.and (d) MG7.

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The calculated values of X for all ring samples are shown in Table1. The results show that theThe values calculated of X valuesin both of the X for tail all and ring muzzle samples are are small, shown indicating in Table 1. a The comparatively results show bad riflingthat the retention values of in X thesein both two theparts. tail and The muzzle values are of small,X in indicating the middle a comparatively of the gun are bad large, implyingrifling retention that the in rifling these retentiontwo parts. in The this values part is of good. X in Boththe middle values of of theSmin gunand areSmax large,reflect theimplying damage that degree the rifling of the retention bore surface, in this wherepart is thegood. larger Both the values values of S are,min and the S moremax reflect serious thethe damagedamage is.degree The of data the in bore Table surface,1 show where that the larger values the of valuesSmin and are,S themax morein both serious the tail andthe damage muzzle areis. The larger data than in Table those 1 inshow the that middle, the values indicating of Smin that and the Smax damage in both degreethe tail and in both themuzzle tail and are muzzlelarger than is obviously those in the greater middle, than indicating that in the that middle. the damage This isdegree consistent in both with the the variationtail and muzzle of bore is diameter. obviously greater than that in the middle. This is consistent with the variation of bore diameter.

Table 1. Experimental values of dmin, Smin, dmax, Smax, and X. Table 1. Experimental values of dmin, Smin, dmax, Smax, and X. Sample dmin/mm Smin/% dmax/mm Smax/% X Sample dmin/mm Smin/% dmax/mm Smax/% X MG1 13.76 8.3 15.31 20.5 2.4 MG1 13.76 8.3 15.31 20.5 2.4 MG3 12.71 0.1 13.33 4.9 49 MG3MG4 12.71 12.73 0.1 0.213.33 13.434.9 5.7 49 28.5 MG4MG5 12.73 12.86 0.2 1.213.43 13.425.7 5.6 28.5 4.6 MG5MG6 12.86 12.94 1.2 1.813.42 13.255.6 4.3 4.6 2.3 MG6MG7 12.94 13.19 1.8 3.813.25 13.314.3 4.8 2.3 1.2 MG7 13.19 3.8 13.31 4.8 1.2 The gun tail shows the highest damage rate, where the bore diameter increases sharply with onlyThe gun a small tail numbershows the of projectiles.highest damage The morphologyrate, where the of MG1bore characterizeddiameter increases by SEM issharply shown with in Figure only a4 .small The number cracks areof projectiles. evenly spaced The morphology on the bore of surface MG1 characterized and are rich in elementsby SEM is S, shown Cu and in Pb. Figure The 4. element The cracks Cu may are evenly come from spaced the on Cu the layer bore of surface bullet shells.and are rich in elements S, Cu and Pb. The element Cu may come from the Cu layer of bullet shells.

Figure 4. SEM morphologies of MG1: (a) bore surface; (b) cracks in the cross-section; and (c) the Figure 4. SEM morphologies of MG1: (a) bore surface; (b) cracks in the cross-section; and (c) the EDS EDS energy spectrum of a crack. energy spectrum of a crack.

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The morphologies of MG2 characterized by 3DIHM are shown in Figure5. The results The morphologies of MG2 characterized by 3DIHM are shown in Figure 5. The results showThe that, morphologies from the left toof theMG2 right characterized (namely from by 3DIHM the part are near shown the tail in Figure to the part5. The close results to show that, from the left to the right (namely from the part near the tail to the part close to MG3),show therethat, from is more the and left moreto the remaining right (namely Cr layer. from Thethe part surface near of the the tail matrix to the near part the close gun to MG3), there is more and more remaining Cr layer. The surface of the matrix near the gun tailMG3), is full there of cracks is more (see and Figure more5a). remaining Cr layer. The surface of the matrix near the gun tail is full of cracks (see Figure 5a). tail is full of cracks (see Figure 5a).

Figure 5. Longitudinal profile morphologies of MG2: (a) the part close to the tail; (b) transitional part; and (c) the part close FigureFigure 5. 5.Longitudinal Longitudinal proﬁle profile morphologies morphologies of of MG2: MG2: (a ()a the) the part part close close to to the the tail; tail; (b ()b transitional) transitional part; part; and and (c ()c the) the part part close close to MG3. toto MG3. MG3. The SEM morphologies of MG3 are shown in Figure 6. The MG3 is at a position where TheThe SEM SEM morphologies morphologies of of MG3 MG3 are are shown shown in in Figure Figure6. 6. The The MG3 MG3 is is at at a positiona position where where the bore diameter is smallest and the rifling retention is best. Only a small amount of Cr thethe bore bore diameter diameter is is smallest smallest and and the the riﬂing rifling retention retention is is best. best. Only Only a a small small amount amount of of Cr Cr layer was peeled off. The EDS spectrum shows that the cracks are rich in S, Pb and Cu layerlayer was was peeled peeled off. off. The The EDS EDS spectrum spectrum shows shows that that the the cracks cracks are are rich rich in in S, S, Pb Pb and and Cu Cu elements. elements.elements.

Figure 6. SEM morphologies of MG3: (a) bore surface; (b) cracks in cross-section; and (c) EDS en- Figure 6. SEM morphologies of MG3: (a) bore surface; (b) cracks in cross-section; and (c) EDS en- Figureergy spectrum 6. SEM morphologiesof a crack. of MG3: (a) bore surface; (b) cracks in cross-section; and (c) EDS energy ergy spectrum of a crack. spectrum of a crack. The cross-sectional morphology of MG7 is shown in Figure 7. The Cr layer on the The cross-sectional morphology of MG7 is shown in Figure 7. The Cr layer on the leadingThe side cross-sectional of the land morphologyline is worn off, of MG7resulting is shown in an inexposure Figure7 .of The the Crmatrix layer which on the is leading side of the land line is worn off, resulting in an exposure of the matrix which is leadingaligned sideto the of Cr the layer land on line the is groove. worn off, The resulting exposed in matrix an exposure also suffers of the from matrix an whichobvious is aligned to the Cr layer on the groove. The exposed matrix also suffers from an obvious alignedwear. The to thedamage Cr layer in this on thearea groove. is primarily The exposed caused matrixby the alsowearing suffers induced from anby obvious bullets, wear. The damage in this area is primarily caused by the wearing induced by bullets, wear.while Thethe peeling damage of in the this Cr area layer is primarily is relatively caused light. by the wearing induced by bullets, while thewhile peeling the peeling of the Cr of layerthe Cr is layer relatively is relatively light. light.

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Figure 7. Cross-sectional SEM morphology of MG7. Figure 7. Cross-sectional SEM morphology of MG7.

TheThe experimental experimental values values of of the the depth depth and and spacing spacing of of the the cracks cracks on on the the bore bore surface surface

ofof MG1 MG1 andand MG5MG5 areare shown in in Table Table 22.. The The Cr Cr layer layer in in MG1 MG1 is iscompletely completely peeled peeled off. off. The TheFigureaverage average 7. Cross-sectional crack crack depth depth SEM of MG1 morphology of MG1 without without of MG7. the Cr the layer Cr layeris 37 μ ism 37 andµm the and average the average crack spacing crack spacingis 40 μm. is 40In MG5,µm. In the MG5, crack the depth crack of depththe matrix of the with matrix Cr layer with is Cr shal layerlow is with shallow an average with valueThe of experimental 32 μm, while values the crack of the depth depth ofand the sp matrixacing of without the cracks the on Cr the layer bore is surface comparatively anof MG1 average and MG5 value are of shown 32 µm, in whileTable 2. the The crack Cr layer depth in MG1 of the is completely matrix without peeled off. the The Cr layer is larger (50 μm). This indicates that the Cr layer can inhibit the propagation of cracks. The comparativelyaverage crack depth larger of (50MG1µ m).without This the indicates Cr layer thatis 37 theμm Crand layer the average can inhibit crack the spacing propagation ofiscrack cracks.40 μm. spacing In The MG5, crack of thethe spacing crack matrix depth ofcontaining the of matrixthe matrix the containing Cr with layer Cr layer theis smaller Cr is shal layer thanlow is smallerwith that an of average thanthe matrix that of with- the matrixvalueout the of without 32Cr μ layer.m, while the In Cr MG7,the layer. crack the In depth damage MG7, of thethe is damagematrixmainly without caused is mainly the by Cr causedwear, layer and byis comparatively wear,the matrix and the has matrix almost haslargerno almostcracks. (50 μm). no cracks.This indicates that the Cr layer can inhibit the propagation of cracks. The crack spacing of the matrix containing the Cr layer is smaller than that of the matrix with- TableTable 2. 2.Experimental outExperimental the Cr layer. values values In MG7, of of crack crack the depthdamage depth and an is d spacingmain spacingly caused in in a cross-sectionala cross-sectional by wear, and matrix. the matrix. matrix has almost no cracks. With or without Cr Average Crack AverageAverage Crack Crack Spac- RatioRatio of Average of Average Spacing RegionRegion With or without Cr Layer Average Crack Depth/μm TableLayer 2. Experimental values of crackDepth/ depthµm and spacing in a cross-sectionalSpacing/ing/μm µm matrix. Spacingto Depth to Depth With With- -Average Crack Spac- - -Ratio of Average Spacing - - MG1RegionMG1 With or without Cr Layer Average Crack Depth/μm WithoutWithout 37 37ing/μm 40 40 to Depth 1.1 1.1 With - 32 - 87 - 2.7 MG5MG1 With 32 87 2.7 MG5 WithoutWithout 37 50 40 92 1.1 1.8 Without 50 92 1.8 With 32 87 2.7 MG5 Without 3.3. Steel Matrix Hardness50 92 1.8 3.3. SteelThe Matrix hardness Hardness of MG1, MG5, and MG7 is demonstrated in Figure 8. From the perspec- 3.3. Steel Matrix Hardness tiveThe of radial hardness hardness of MG1, distribution, MG5, and MG7the hard is demonstratedness of the matrix in Figure near8. Fromthe bore the perspec-surface is The hardness of MG1, MG5, and MG7 is demonstrated in Figure 8. From the perspec- tivelower of radial than that hardness near the distribution, edge. From the the hardness tail to the of the muzzle, matrix the near hardness the bore shows surface an is increas- lower tive of radial hardness distribution, the hardness of the matrix near the bore surface is thaning trend. that near the edge. From the tail to the muzzle, the hardness shows an increasing lower than that near the edge. From the tail to the muzzle, the hardness shows an increas- trend.ing trend. 390 390 380 MG7 380 MG7 MG5 370 MG5 370

MG1 MG1 360360 Hardness/HV Hardness/HV 350350 SofteningSoftening

340340

0246810121402468101214 Distance/mm Distance/mm FigureFigure 8. HardnessHardness of of MG1, MG1, MG5, MG5, and and MG7. MG7. Figure 8. Hardness of MG1, MG5, and MG7.

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4. Discussion 4.1. Damage Characteristics of Bore Surface The damage mode of the gun barrel varies along the axial direction. This is because the acting conditions such as temperature, stress, and corrosion on the bore surface are constantly changed along the axial direction. At the tail of the barrel, the damage mode of the bore surface is as follows: the Cr layer falls off, with the exposed matrix being covered with crack networks. This phenomenon was also observed by Li et al. [18] in a hot-stamping boron steel with Al–Si coating. Cyclic high temperature and pressure caused by shooting leads the Cr layer to compress and yield, and brittle cracks occur during the cooling process [19]. With the cyclic tensile loading, low cycle fatigue failure occurs [20–22]. According to the stress analysis based on the linear elastic theory, when the Cr layer is under a load and the maximum tensile stress is close to the yield strength of the base steel, the Cr layer is easy to crack [23]. The tensile stress σ in the Cr layer can be estimated according to the following formula:

E0 σ = ( )P(r 2 + r2)/(r 2 − r2) (4) E 2 1 2 1 0 where E and E are the elastic moduli of chromium and barrel steel, respectively, r1 is the bore radius, r2 is the outer radius of the barrel, and P is the gas pressure caused 0 2 2 2 2 by gunpowder. For a 12.7 mm machine gun: E /E ≈ 1.5, (r 2 + r1)/(r 2 − r1) ≈ 1.27, P = 310 MPa, thus σ ≈ 591 MPa. The strength of gun steel is 198–600 MPa at 500–700 ◦C. Within this stress range, the Cr layer is most likely to crack. Under the condition of high pressure, the gas produced by gunpowder promotes the cracks to propagate continuously. The EDS energy spectra of cracks conﬁrmed this phenomenon (see Figures4 and6). Such damage prevails in the tail and the middle of the barrel. At the muzzle, the cracks in the Cr layer are difﬁcult to extend to the matrix because of the reduced pressure. The width, depth, and density of cracks affect the variation of the bore diameter. The ratio of the average crack spacing to crack depth in the tail and middle parts ranges between 1.1 and 2.7. This is similar to that obtained by Underwood J.H. et al. [11], which is 2. During the cooling process after shooting, the residual tensile stress is formed in the coating. Due to the formation of the initial main cracks, the macro stress caused by the residual tensile stress is reduced, and it is in balance with the interfacial shear stress in the L-length range. According to slip zone theory, Underwood et al. analyzed the cracking of the Cr layer, and gave a mathematical expression of crack depth h and crack spacing L:

L S Sy = resid = (5) h τy τy

In the formula, Sresid is the residual tensile stress, τy is the interfacial shear stress, and Sy is the tensile yield strength. For the Cr layer, the general yield strength Sy is twice that of the shear stress τy, thus the value of L/h is approximately 2. In the middle part of the barrel, the Cr layer was partially peeled off, with the bore diameter varying only slightly. Compared with the condition in the tail, both the temperature and pressure in the middle of the barrel were reduced. Thus, the damage of the Cr layer is less and the damage rate slows down. Qiao et al. [15] believed that the middle part of the barrel was an important area for projectile rifling etching and has a significant impact on shooting accuracy. The damage characteristics in the muzzle part are as follows: the Cr layer on the land lines are worn off, and the bore diameter increases with a high rate. With the projectile volume increasing from 0 to 24%, the bore diameter of the muzzle increases from 12.67 to 12.81 mm. This increasing rate is much faster than that in the middle part. The muzzle is the final place where the linear and angular velocities of the projectile reach their highest Metals 2021, 11, 348 9 of 10

value, thus there is severe friction between the warheads and the bore surface. The Cr layer on the land lines in the muzzle was completely worn off, which results in a low rifﬂing retention of X = 1.2 and weakens the guiding effect on projectiles. The damage characteristics of the barrel are summarized as follows: (i) the main damage mode of the Cr layer from the tail to the muzzle varies from peeling to wearing; and (ii) the bore diameter in both the tail and muzzle increases with a high rate, while the damage rate in the middle part is comparatively low.

4.2. Softening of Steel Matrix Near Bore Surface From the gun tail to the muzzle, the hardness increases gradually. The reasons for the softening of the tail matrix are as follows: (1) during the processing of barrel and after Cr plating, a stress–relief–annealing is generally needed for the gun tail, with temperatures between 500 and 600 ◦C and a soaking time of 1–3 h; (2) the gun tail suffers from the highest temperature in service. Feng et al. [24] calculated the bore surface temperature for a similar gun barrel (12.7 mm machine gun). Their calculation shows that after 120 shots, the bore surface temperature reaches 827 ◦C and then attenuates to 594 ◦C in 10 ms. Shooting temperature is also an important reason for the softening of the matrix. From the bore surface to the outer surface of the barrel, the hardness increases gradually, which on the other hand, conﬁrms that the shooting temperature causes the matrix to soften. According to the high bore surface temperature and pressure, it can be inferred that the bore surface was subjected to plastic deformation. Li et al. [25] declared that the bore surface of a rapid-ﬁre artillery barrel suffers from plastic deformation along the radial direction within a thickness of about 0.467 mm. Under the action of high thermo- compression coupling stress, the bore surface softens. This increases the tendency of the plastic deformation of the matrix near the bore surface and increases the bore diameter, which will lead to a reduction in shooting accuracy.

5. Conclusions In this study, the damage characteristics of a machine gun barrel were systematically investigated. The main results are as follows: 1. The bore surface of the barrel presents different damage characteristics along the axial direction. In the gun tail, the Cr layer falls off completely, leaving the exposed matrix full of crack networks. The steel matrix without the protection of a Cr layer exhibits a higher damage rate. In the middle part, the Cr layer is well retained, with only a small part peeling off. This part of the barrel has a relatively small damage rate. In the muzzle, the Cr layer on the land lines is completely worn off, leaving the matrix exposed. The muzzle also exhibits a high damage rate. 2. The matrix close to the bore surface is softened due to the high temperature and pressure caused by shooting. The hardness in the tail part is lower than those in both the middle and muzzle parts because the tail is subjected to higher temperatures during both stress–relief–annealing and ﬁring processes. The softening of the matrix increases the tendency of the plastic deformation of the bore surface and increases the bore diameter, which will lead to a reduction in shooting accuracy.

Author Contributions: Z.-m.W.: formal analysis, writing—original draft; C.-d.H.: writing—review and editing, supervision, methodology; Y.-x.W.: investigation; H.-c.L.: review & editing, supervision; J.-s.L.: methodology; H.D.: methodology. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Bottleneck Project of General Armament Department (grant number 30407). Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Metals 2021, 11, 348 10 of 10

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