【導(dǎo)讀】Ferrite(?Austenite(?),isasolidsolution,thatis,the. (seefigure1).

　　

【正文】 ulfide, which has a melting temperature of 1814176。F. This is why manganese is added to steel. It bines with the sulfur to form manganese sulfide. Some steels called free machining steels have a high sulfur content of around %. Manganese is added to these highsulfur steels because it helps the steel to be free cutting. The high manganese bines with the high sulfur to form the manganese sulfide pound. This type of steel is then cold drawn through dies, making the manganese sulfide elongated or needlelike and easy to machine. Phosphorus. This element is also an impurity. It does have the ability to strengthen steel but at the expense of ductility. Maximum percentage should be approximately %. Pg: 21/ 69 Fig. 8 The cubic lattice structure of ferrite and austenite with carbon in solution. Pg: 22/ 69 Acicular ferrite Acicular consists of a fine structure of interlocking ferrite plates. Acicular ferrite is formed in the interior of the original austenitic grains by direct nucleation from the inclusions, resulting in randomly oriented short ferrite needles with a 39。basket weave39。 appearance. This interlocking nature, together with its fine grain size ( to 5um with aspect ratio from 3:1 to 10:1), provides maximum resistance to crack propagation by cleavage. Acicular ferrite is also characterized by high angle boundaries between the ferrite grains. This further reduces the chance of cleavage, because these boundaries impede crack propagation. It is reported that nucleation of various ferrite morphologies is aided by nonmetallic inclusion。 in particular oxygenrich inclusions of a certain type and size are associated with the intragranular formation of acicular ferrite. Acicular ferrite is a fine Widmanst228。tten constituent, which bitch is nucleated by an optimum intragranular dispersion of oxide/sulfide/silicate particles. Composition control of the weld metal is necessary in order to maximize the volume fraction of acicular ferrite, because excessive alloying elements can cause the formation of bainite and martensite. Figure 2. (a) Effect of different speeds of nucleation and growth on formation of pearlite colonies。 (b), (c), (d) diagrammatic representation of formation of pearlite, upper bainite and lower bainite Pg: 23/ 69 Figure: An illustration of the essential constituents of the primary microstructure of a steel weld deposit. The diagram is inaccurate in one respect, that inclusions cannot be expected to be visible in all of the acicular ferrite plates on a planar section of the microstructure. This is because the inclusion size is much smaller than that of an acicular ferrite plate, so that the chances of sectioning an inclusion and plate together are very small indeed. Widmanst228。tten ferrite w, acicular ferrite a Non metallic inclusion in steel Pg: 24/ 69 Figure 1. Transmission electron micrograph shows the inhomogeneous nature of inclusions. Figure 2. Micrograph showing the nucleation of acicular ferrite platelet on an inclusion in the laser weld. Pg: 25/ 69 More micrographs Figure 9. Forms of carbide in microconstituents in steel Pg: 26/ 69 Widmanst228。tten ferrite Ferrite Pg: 27/ 69 Figure 2. Austenite (gamma iron) crystal structure Figure 3: Photomicrograph of austenite, 325X (Callister, 1994). Figure 2. Ferrite (alpha iron) crystal structure Pg: 28/ 69 Figure 2: Photomicrograph of alpha ferrite, 90X (Callister, 1994). Figure 4: Photomicrograph of a wt% C steel having a microstructure consisting of a white proeutectoid cementite work surrounding the pearlite colonies, 1000X (Callister, 1994). Figure 3. Pearlite microstructure (Light background is the ferrite matrix, dark lines are the cementite work) Pg: 29/ 69 Ferrite + bainite (Ferrite + martensite will look similar) Figure 3. (a) Lathe martensite formed in 0,08176。C steel quenched in brine from 100176。C (x20200), b) Twinned martensite in Fe30%Ni (x110000) Pg: 30/ 69 Figure 1. Microstructure of same steel showing part of ferrite work, Widmanst228。tten and feathery structure. Ferritewhite. Pearlitedark ( x 80) Figure 2. Macrostructure of cast steel revealing large prirmary austenite crystals due to presence of impurities (x 4) Figure 3. Same steel imperfectly annealed ferrite formed in masses outlining original cast structure (x80) Figure 4. Same steel properly annealed: ferrite and perlite uniform and fine (x80) Figure 5. As cast: cementite work and plates in pearlite (x 100) Figure 6. Heated to 1050176。C and quenched in water. Large grains (x 100) Pg: 31/ 69 Figure 7. Cementite globules in properly hotworked steel (x 200) Figure 8. Cementite globules in martensite, in hardened steel (x 200) Figure 12. Martensite and quench crack. Steel (C, 0,5) quenched in water from 900176。C(x400) Figure 13. Nodular troostite in martensite (x400) Figure 12. Martensite and quench crack. Steel (C, 0,5) quenched in water from 900176。C(x400) Figure 13. Nodular troostite in martensite (x400) Pg: 32/ 69 Figure 14. Sorbite in quenched and tempered (600176。C) steel (C, 0,5) (x500) Figure 15. Casehardened screw. Cracked martensitic case (white), martensite and ferrite core (x30) Before Spheroidizing After Spheroidizing The microstructure of a 1065 steel showing the structure as received from the steel mill. The structure is a fine pearlite produced through rapid cooling. (500X) The microstructure from the same 1065 st