III. Chuyen bien xay ra khi giu nhiet :

Khi giu nhiet khong co chuyen bien nao khac voi khi nung nong. Tien hanh giu

nhiet nham cac muc dich sau :

-Lam dong deu nhiet do tren toan tiet dien, de cho loi cung co chuyen bien nhu o be

mat

-Co thoi gian de hoan thanh cac chuyen bien khi nung nong.

-Lamdong deu thanh phan hoa hoc cua austenit.

-Thoi gian gu nhiet chi nen vua du khong nen keo dai qua muc lam cho hat lon.

IV.CHUYEN BIEN CUA OSTENIT KHI LAM NGUOI CHAM:

Lam nguoidang nhiet va lam nguoi lien tuc.

1.cac san pham cua su phan hoa dang nhiet cua ostenit qua nguoi:

Tu gian do chu C ,ostenit khi lam nguoi xuong duoi 727 do C chua chuyen bien ngay ma ton tai 1

thoi gian nhat dinh truoc khi chuyen bien,phan hoa dc goi la ostenit qua nguoi.Tren gian do chu C:

-o ten 727 la khu vuc ton tai ostenit on dinh

-ben trai duong cong C thu nhat la ostenit qua nguoi

-khoang giua 2 duong chu C la ostenit chuyen bien

-ben phai duong cong chu C thu 2 la cac san pham phan hoa dang nhiet cua ostenit qua nguoi.

-Duoi duong Md la ostenit va mactenxit du

*san pham phan hoa dang nhiet:

- khi phan hoa sat o A1(tren duoi 700 do C),voi do qua nguoi nho denta T khoang 25.Hon hop

pherit-xematit tam tao thanh voi kich thuoc tho to,khoang cah giua cac tam khoang 0.001 mm

,do cung 10-15hrc goi la pedlit tam.

-khi ostenit phan hoa o nhiet do thap hon(khoang 650,voi denta T xap xi 75),hon hop pherit-xementit

tam tao thanh nho min hon,khoang cach giua cac tam co 0.25-0.3 nghiromet,ko phan biet dc chung tren

kinh hien vi,co do cung 25-35hrc,goi la xoocbit toi.

-khi cho ostenit phan hoa o nhiet do thap hon(khoang 500-600)hon hop tao thanh se nho min hon nua,

khoang cach giua cac tam khoang 0.1-0.15 nghiromet,do cung co 40hrc goi la troxit toi.

-khi ostenit phan hoa o nhiet do thap hon nua,tuc la thuoc ve nua duoi cua duong cong chu C(450-250)

hon hop nhan dc rat nho min,do cung 50-55hrc goi la bainit.

2.su phan hoa ostenit khi lam nguoi lien tuc:

a.voi cac toc do nguoi khac nhau,ostenit se bi hoa nguoi den cac nhiet do khac nhau va phan hoa thanh

cac san pham tuong ung voi nhiet do do

-lam nguoi cham cung lo(V1),vecto nguoi cat duong cong chu C o sat A1 san pham phan hoa la peclit tam voi do cung thap nhat.

-lam nguoi trong ko khi tinh(V2),ostenit phan hoa thanh xoocbit.

-lam nguoi trong ko khi nen(V3),vecto nguoi cat chu C tai phan loi,ostenit phan hoa thanh troxtit.

-lam nguoi trong dau(V4) vecto nguoi cat duong cong chu C thu nhat tai phan loi,ostenit chi co mot phan chuyen bien thanh troxtit

phan con lai la mactenxit nen sp la trostit va mactenxit

-lam nguoi nhanh hon nua(trong nc lanh voi V5),vecto nguoi ko cat duong cong chu C,phan lon ostenit qua nguoi chuyen bien thanh mactenxit.

b.to chuc thuong ko dong nhat tren toan tiet dien,nhat la trg hop tiet dien lon vi be mat lon nhanh hon loi,

c.ko nhan dc hoan toan to chuc bainit.Trong mot so truong hop thay to chuc bainit hinh thanh cung troxtit va mactenxtit