AN INVESTIGATION OF THE NEEDLE LIKE STRUCTURE IN HIGH SPEED STEEL. Thesis for the Degree of Met. E; \Valter G. Hildorf I 9 2 7 . . THESIS r. L» ‘l a ' = ’f I he I 2 f" '”*' (‘55.1‘3‘ . .r *y“—|fi:uo"‘i (r: 1 -.' -'0 m“ I. r . '.1;..f4;..+-1_ , ~ '.4r ~-: , fi2%?'f.£fi '1," .. . ‘ a . h-_" ‘ ‘ ‘< ‘7: ' l‘..~'.~_ { .. . .‘J I ~ ‘ IO- -' . 4 f__ . “‘7' h .«In- ~ A .. .l —, v.7 . b _ . I . ._‘u ‘. r. ’4' If; 39¢. VT." - villi ‘ 3.x“ "EMT ‘ j «r -— ¢. . '_ .4", ‘V ‘O”? A:-'.|-»' ~)' ‘3‘? ' ;*7P:-‘.5"'~1-.$, )r . '0; "k; _uq '4" {g 3"}- I r 7| ‘fl > .- I‘ _. b',“\¥,h,-.._ - vl.- - . I}: "9:? , . .. .‘ tau“ . . J .ff .-.b.'§"_1.':¥'- . _ ’ Yr ‘, r . ‘$.f.‘ J ‘ -5. ‘ o W .2 “55' fit -'..¢J’_}‘« a:.>.(‘c;. ‘ h}: ' J, I ‘ ’ I p." I‘ I I 3 0 4:96. “39“,; ‘ , ,1) ‘7’ "i _ .:° ‘.)A’xi‘v:\? ‘,I.‘ f 'r t ‘ 09.- :- "hwy “j," W”: 23'. E.“ ‘; . L. S m_, if "Q‘"‘- 5 .- ‘S 5-3: ""291. ' ' . - '1. ‘ ~--. . - . ' 4' w . . 1'.“ :31}: n f,- lfiff-J 3 . _ (If .- 4 Ir: AN INVESTIGATION OF THE NEEDLE LIKE STRUCTURE IN HIGH SPEED STEEL A Report Submitted To The Faculty Of The Michigan State College by ’ -‘. I Walter G? Hildorf Candidate For The Degree 0f Metallurgical Engineer June 1927 m III (II AN INVESTIGATION OF THE NEEDLE LIKE STRUCTURE IN HIGH SPEED STEEL CONTENTS I. IntTOdUCtion 000.00.00.0000000000000000000.0000... 4 1. Characteristics of this structure ........ 4 2. Cause of this structure is unknown ....... 4 5 3. Theories as to its cause.................. II. Purpose of the investigation ..................... 5 III. Samples and their heat treatment ................. 6 1. Size and choice of samples ............... 6 2. Heat treatment and metallOgraphio examination .eeeeeeoeeeeeeeeeeeeeeeeeeeeee 6 (a) Hardening .OOOOOOOOOOOOOOOOOOOOOO. 6 (b) Metalloaraphic inspection of hardened samples Cooeeeeeeeeeeeeeo 7 (C) Tampering .oeeeoeoeeeeeeeeeeoeoeeo 7 (d) metallOgraphic inspection of tem- pered samples .OOOOOOOOOOOOOOOOOOO 8 IV. Description of microstruotures ................... 1. Typical needle like structure ............ 2. Series of hardened structures ............ ‘0 (D a: CD 3. Series of tempered structures ............ 101166 I V. VI. VII. VIII. ROCk“’7ell harmless OOOOOOOOOOOOOOOOOOOOOO00.0.0.0... 1. HPTdened 88mD198 .0eoooooooooooeooooooeoooo 2. Temnered samples eeee00000000000000.0000... Results from other investigations of this StruCture 00000000000000.eoooeeooeeeeeeeeoeeoeeeoeo One method of orercomina this structure ........... Summary 0..COO...OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 24 24 25 AN INVESTIGATION OF THE NEEDLE LIKE STRUCTURE IN HIGH SPEED STEEL. INTRODUCTION CHARACTERISTICS OF THIS STRUCTURE The needle like structure, so called because of its simi- larity to a myriad of needle points has been.under investigation at the laboratory of the Rec Motor Car Company for some time because it has been found in practically all high speed steel milling cutters, reamers and other tools which have failed due to breakage of the teeth or crumbling of the cutting edges. This structure seems to make the steel very brittle. In most cases it exists as a thin layer on the outside of the tool, often not over .005" deep which will soon crumble away and the tool will not cut smoothly nor to size. If this structure extends very deep - it has been found to extend completely through some milling cutters and reamers - the result is quite often that the edges will wear away quickly and then the additional load will cause the teeth to break, ruining the tool. CAUSE OF THIS STRUCTURE IS UNKNOWN The exact cause of this structure or a heat treating practice by which it may be avoided appears to be unknown, although its bad effects are often eliminated by allowing enough stock on the tools so that this structure can.be ground away. A fairly accurate survey of the literature on metallurgical subjects revealed only three papers in which this structure was mentioned.’ Dr. C. A. Edwards states that "A new brittle constituent appears at about 12920F. in tempering which caused the failure of high speed tools". In a later paper published Jointly with :ur. H. Kikkawa he abandons this position. Dr. J. A. matthews refers to the "brittle constituent" mentioned in Dr. Bdwards' paper and states "This constituent is due solely to overheating and is not produced when tempering at 12920F. in properly hardened high speed steel". THEORIES AS TO ITS CAUSE Other theories have been advanced such as: l. Uneven heating.’ 2. Insufficient tempering. 3. Hardening temperature too low and not allowing the tools to cool to a low enough temperature in the quench befbre they are tempered. 4. That the structure is martensite. PURPOSE OF THE INVESTIGATION Since this structure does not occur regularly and since no t The Iron Age - July 3, 1919. one has proven how it can be produced or avoided it was decided to make this investigation as simple and as free from complica- tions and a large number of variables as possible and yet help to prove or disprove one or more of the above theories. SAMPLES AND THEIR HEAT TREATE‘N- SIZE OF SAMPLES _ Two samples of high speed steel 1/2 in. x 1/2 in. x 18 in. long were chosen from a steel company who furnish about 85% of the high speed steel used.by the Rec Motor Car Company. These bars were chosen because we have had more experience with fllis steel than with other steels and because it seems to be fairly free from this brittle structure. This, however, might be due to our method of heat treatment rather than to the steel itself. HEAT TREATMENT AND METALLOGRAPHIC EXAMINATION HARDENING The samples were preheated at 1550°F. and then the bars were placed in the furnace so that about 12 in. was inside the door or in the furnace preper, while the balance of the bar was out~ side the door. The furnace was gas heated having a heating chamber 8 in. x 11 in. x 13 in. This furnace was maintained at -a temperature of 25000F. by means of a Brown pyrometer equipment. frhe samples were in the furnace approximately four'minutes. The mention. The overheating extended to about Fig. 11-3. This is shown by the size of the austenite grains and also by the fact that the carbides have gone to the grain boundaries. In all samples up to Fig. 14-3 the carbides were more or less in the shape of cubes, while in the balance of the samples they were more nearly spherical in shape. The first indication of incom- plete austenite grains is shown in Fig. l9-H which indicates that the temperature at this point was not high enough for the forma» tion of complete grains. SERIES OF TEMPEREIISTRUCTURES This series shows the tempered structures of the samples above mentioned. They are designated by Fig. l-D to and includ- ing Fig. 21-D. The tempered samples showed no needle like structure in the first sample, Fig. l-D, but it was very evident from Fig. 2-D to and including Fig. l7sD. The depth of the needle like structure varied from .005 in. in Fig. 2-1) to .0005 in. in Fig. 17-1). This series of photOgraphs also shows several very interesting features, ‘Which probably have no bearing on the needle like structure, but are of general interest. The carbides exist in the form of cubes and also in narrow irregular shaped bands from Fig. l-D to Fig. 11-D, beyond this they exist mainly as spheres. ' The martensitic structure is present in a large number of the Samples for example Fig. 2-D shows this structure. A large num- ber of the samples show the outlines of the austenite grains. 10 This is rather remarkable, because the samples were drawn fer 14 hours. ROCKWELL HARDNESS The Rockwell hardness was taken.of both the hardened and the tempered samples. The hardness is shown under each of the structures. // ”NOIQ [3N0 .uh. fix tax: 3 0h. .\.k.k.u> 0.05th hot... up 991th lark lle-i IU .thflkh. Qnflmlh. \‘Vht k0 Welsh-h .>\ Q\Ln \\ W\.k.h..W\~ >\\ “(cab {\ thb hm. Stealth. deKB QWHVOXW \\ W\\\ 12 Fig. 1. This shows a typical needle like structure. It is believed that this structure is not martensite. Compare it with Figs. 2-D and 5-D and also with martensitic structures as shown in The Metallography of Steel and Cast Iron by H. M. Howe and in The Metallography and Heat Treatment of Iron and Steel by Alb 6 It 3.1170111. magnification --------- -- 10001 Etched with a'5% solution of nitric acid in alcohol. Fig. l'H Flee I‘D Rockwell hardness ----- 62.6 Rockwell hardness ----- 62.6 Depth of needle like Depth of needle like structure ------------- None structure - ------------ None Badly overheated Badly overheated Pig. 2-H Fig. 2-D Rockwell hardness ----- 64.8 Rockwell hardness ----- 62.6 Depth of needle like Depth of’needle like structure ------------- None structure --- --------- - .oosw Carbides are mainly in the Structure is martensitic grain boundaries Magnification ----- 10001 Etched with 5% nitric acid in alcohol 14 Rockwell hardness ----- 64.5 Rockwell hardness ----- 62.8 Depth of needle like Depth of needle like structure ----------- -- None structure - ------------ .004" Structure is martensitic Pig. 4-H rig. 4-D .Rockwell hardness ----- 63.8 Rockwell hardness ----- 61.5 Depth of needle like Depth of needle like structure ------------- None structure ------------- .0025" The outline of the austenite grains is still present Magnification ---_-- 10001 Etched with 5% nitric acid in alcohol 15 Rockwell hardness ----- 64 Rockwell hardness ----- 62.8 Depth of’needle like Depth of’needle like structure ------------ - None structure ------------- .002” F180 6-H F180 6'D Rockwell hardness ----L 64.6 Rockwell hardness ----- 62 Depth of needle like Depth of needle like structure ------------- none structure ------------- .003" Magnification ----- 10001 Etched with 5% nitric acid in alcohol 16 Figs 7-H F18. 7-D Rockwell hardness ~---- 64 Rockwell hardness ----- 62.8 Depth of’needle like Depth of needle like structure ------------- Bone structure ------------- .002" \, '\ \\ _ __ J, Rise 8-3 Fig. 8-D Rockwell hardness ----- 66.6 Rockwell hardness ----- 61.8 Depth of needle like Depth of needle like structure ------------ - Rene structure ------------ - .002” magnification -~--- 1000! Etched.udth 5% nitric acid in alcohol 17 F16. 9-H Fig. 9-D Rockwell hardness ----- 64.2 , Rockwell hardness ----- 65.1 Depth of needle like . Depth of needle like structure -- ----------- None ’structure ------------- .002" . a Fig. 10-D Rockwell hardness —---- 64.6 Rockwell hardness ----- 65.5 Depth of’needle like Depth of’needle like structure ------------ - None structure -----~- ----- - .002" Imagnification ----- 10001 Etched with 5% nitric acid in alcohol Fig. 11-3 Rockwell hardness ----- 65 Depth of needle like structure ------------- None Correct hardening temperature Figs 12"H Rockwell hardness ----- 65.1 Depth of’needle like structure ------------- None Fig. 11- H Rockwell hardness ----- 52.3 Depth of needle like structure -- .......... - .004» Note the martensitic structure Pig. lBa-D Rockwell hardness ----- 53 Depth of needle like structure ............ - .002“ luagnification ------ 10001 Etched with 5% nitric acid in alcohol 19 \ \ \ ‘ x \ \\ ’\_‘ i Fig. lS-H ' Fig. 13-D Rockwell hardness ----- 64.5 Rockwell hardness ----- 62.5 Depth of needle like Depth of needle like structure ------------ - None structure ------------- .002” Fig. 14.3 P18. 14-” Rockwell hardness ----- 65.1 Rockwell hardness ----- 62.1 Depth of needle like Depth of needle like structure ------------- None structure ------------- .0005" Magnification ----~ 10001 Etched with 5% nitric acid in alcohol 3180 15-1) Rockwell hardness ----- 65.8 Depth of needle like structure ------------- None Note large number of very small carbides Rockwell hardness ----- 61.5 Ibpth of needle like structure ------------- .0005" Fig. 16-11 Fig. 15-1) Rockwell hardness ----- 65.1 Depth of needle like structure ------------- None Rockwell hardness ----- 61.5 Depth of needle like structure ------------- .0005” ZMagnification ---- 1000K Etched with 6% nitric acid in alcohol 21 Fig. IVOH Fig. 17-1) Rockwell hardness ----- 64.5 . Rockwell hardness ----- 60 Depth of needle like Dapth of needle like structure ------------- None structure ------------- .0005" Nearly all carbides are spherical Figs 18-3 F18 e 18-D Rockwell hardness ----- 64.1 Rockwell hardness ----- 59.6 Depth of needle like Depth of’needle like structure ------------~ None structure ------------- None ‘Magnification ----- 10001 Etched with 5% nitric acid in alcohol Fig. 19"H Rockwell hardness ----- 65.5 Depth of needle like structure ------------- None Incomplete austenite grains Fig. 20-3 Rockwell hardness ----- 62.8 Depth of needle like Structure ------------- None 22 Fig. 19-1) Rockwell hardness ----- 58 Depth of needle like structure 9 ------------ None Rockwell hardness ----- 56.5 Depth of’needle like structure ------------- None 'Magnification ----- 10001 Etched.with 5% nitric acid in alcohol Figs 21-3 Rockwell hardness ----- 58 Depth of needle like structure --------~---- None No definite austenite grains. Temperature too low for the formation of‘austenite grains. .‘i'; c' ‘ . i g .‘ ' ,O ,V‘ "s ’ . s .. 1.. s Fig. 21-1) Rockwell hardness ----- 52.5 Depth of needle like structure -------—----- None Structure very similar to the hardened structure. 24 RESULTS FROM OTHER INVESTIGATIONS OF THIS STRUCTURE In a series of tests, to determine the effect of varying the length of the tempering time upon the structure and impact value of quenched high speed steel, each sample was quenched from as near 2500°F. as possible and than tempered at various times, varying from 15 minutes to 15 hours. The temperature was 1070 to 1080°F. Every tempered sample showed the needle like structure. It varied in depth from .010 in. in the sample tempered far 12 hours to .0007 in. in the sample drawn fer 9 hours. There ap- peared to be no connection between the length of the tempering time and the depth of the needle like structure. For example it was .0025 in. deep in both the sample tempered for 15 min. and the sample tempered for I hours. Other investigations, in- cluding the present one, seem to indicate that a variation in the tempering time or temperature has no influence in causing _this structure. one meson or owncomne THIS STRUCTURE Since this structure is usually not over .005 in. deep it can usually be removed by leaving enough stock on both sides of the cutting edges so that .005 in. may be ground away after .hardening and tempering. However, this is difficult and expen- sive and in some cases even impossible without special grinding machines. 25 SUMMARY In conclusion, the results of this and our other investiga- tions would seem to warrant the fbllowing statements: 1. 2. That overheating might not be entirely the answer because this structure was present as far as Fig. 17~D, while Fig. ll-D showed about the correct hardening temperature. In other investigations where the temperature was held as near 25000F. as possible this structure was sometimes present. However, overheating or some one temperature not necessarily very high considering high speed steel temperatures probably has a decided in- fluence, if it is not the entire cause, because in most cases this structure is on the outside of the samples, which be- comes the hottest, also this structure is always very coarse which helps to prove the above statement. That uneven heating is out of the question because it is often.present in small samples which have been heated with labora- tory accuracy. 3. 4. 5. 26 Insufficient tempering is not the cause for the-reason that the samples in this investigation were tempered for 14 hours, which is many times the usual length of thme. In a previous investigation where the tempering time was varied, there seemed to be no effect so far as time was concerned. Tempering before the tools were entirely cold, certainly was not the cause of the needle like structure in this case, for the reason that the samples were at room temperature for two days before they were tempered. The structure does not appear to be a mar- tensitic structure because they do not look alike, and.because it seems to bear no re- lation to tempering time nor temperature. Martensite is a transitory structure which should vary with both time and temperature. Dr. Edwards and Dr. Matthews both refer to it as though it was a new structure. In fact Dr. Edwards calls it "a new brittle constituent". 6. 27 The infbrmation detained in this and in our other investigations, does not def- initely prove the cause of Uiis structure. It does, however, show one method of over- coming the difficulty where it exists as a thin layer on the outside of'tools. It also shows that tempering time nor temper- ature seem to have no effect upon it and that it is very likely caused in some way during the hardening Operation, therefbre a practice of hardening should be used which.would eliminate this structure as much as possible. RBOM USE ONLY "'7mmvigavvngmmn'E