V.CHUYEN BIEN OSTENIT KHI LAM NGUOI NHANH(CHUYEN BIEN MACTENXIT):

-khi lam nguoi nhanh ostenit voi van toc lon hon hoac bang van toc nguoi toi han Vth(ko cat duong cong chu C),thi ostenit k kip phan hoa thanh hon hop

ferit+xementit theo co che khuyech tan ma bi qua nguoi xuong tan nhiet do Md va chuyen bien thanh mactenxit

-mactenxit la dd ran qua bao hoa cua Cacbon trong sat anpha-Fe anpha voi nong do cacbon bang do cacbon cua ostenit,kieu mang chinh phuong the tam

co do cung cao.

-dac diem cua chuyen bien mactenxit:

a.chi xay ra khi lam nguoi nhanh ostenit voi toc do lon hon hay bang toc do nguoi toi han.Chuyen bien mactenxit ko xay ra khi nguoi dang nhiet.

b.la chuyen bien ko khuyech tan,nguyen tu C van ju nguyen vi tri,chi co nguyen tu sat chuyen vi de tao ra mang chinh phuong tam khoi

,khoang cach dich chuyen ko wa 1 thong so mang.

Giua 2 kieu mang ostenit va mactenxit co quan he dinh huong xd sao cho cac mat va phuong day dac cua chung song song voi nhau.

c.la qua trinh tao ra ko ngung cac tinh the moi voi toc do rat lon(1000-7000 m/s)

d.chuyen bien chi xay ra trong khoang nhiet do xd nhiet do bat dau chuyen bien (Md) den nhiet do ket thuc (Mk).Vi tri 2 nhiet do nay ko

phu thuoc vao toc do lam nguoi,chi phu thuoc vao thanh phan cacbon

va nguyen to hop kim trong thep.

e.chuyen bien xay ra ko hoan toan:khi lam nguoi cang gan den Mk luong mactenxit sinh ra cang nhieu,nhg ko the dat dc 100%,con ostenit du.

Khi chuyen bien mactenxit thanh ostenit the tich se tang len

(VM>V gama).luong ostenit du phu thuoc vao:

- vi tri cua Mk:diem Mk cang thap thi ostenit du cang nhieu(thap hon 20 do C)

-luong C trong mactenxit cang nhieu the tich rieng cang lon.

VI.CHUYEN BIEN XAY RA KHI NUNG NONG THEP DA TOI(KHI RAM):

a.giai doan I(<200 do C):

-khi nung nho hon 80 chua xay ra j,to chuc van la mactenxit va ostenit du.

-tu 80-200: ostenit du chua chuyen bien.Luc nay mactenxit co su tap trung C lai va tiet ra duoi dang cacbit co cong thuc FexC(X tu 2-2.4)

o dang tam mong va rat phan tan.

luong C trong mactenxit giam tu 0.8% xuong con 0.25-0.4% va ty so c/a giam.Cacbit chua co kieu mang rieng,ton tai trong kieu mang mactenxit.

Hon hop cacbit va mactenxit ngheo C con la mactenxit ram.

Fe anpha(C)0.8-->[Fe anpha(C)0.25-0.4 +Fe2-2.4C]

cuoi gia doan nay to chuc cua thep la mactenxit ram va ostenit du.

b.gd II(200-600)

c.gd 3(260-400)

d.gd4(>400)