A cold roll can pass every dimensional check and still fail from the inside. Hydrogen flakes are internal cracks that open after forging while the roll cools. Chinese mill practice also calls them white spots. They leave no mark on the surface. Ultrasonic testing finds them. Sin embargo, it works only when the operator reads two signals correctly: the loss of back-wall echo and the position of the defect echoes. The sections below cover how hydrogen flakes form in a roll forging. They also cover how inspection separates them from inclusions, porosity and segregation.
What Hydrogen Flakes Are
A hydrogen flake is an internal crack caused by hydrogen. Hydrogen dissolves in liquid steel, and it stays in the steel as a solid solution once the ingot solidifies. At that stage the steel holds hydrogen in a supersaturated state. If a solidified but still hot ingot goes to rolling or forging, hydrogen starts to precipitate. Precipitation begins during the cooling that follows hot working. It diffuses first to voids in the metal and builds pressure there.
Slow cooling, or a small ingot section, lets hydrogen diffuse out, so no serious harm follows. Fast cooling gives hydrogen no time to escape. Flakes can then form as the steel passes through the 250 °C a 100 °C band.
Identifying a flake means analysing a dense and dispersed defect population. The signals below separate it from other indications.
Reading the Drop in Back-Wall Echo
Inspection normally uses a 2 MHz probe at Φ2 mm defect equivalent sensitivity. When a roll carries hydrogen flakes, many defect echoes appear at once, the pattern operators call a forest of echoes. Widen the scan range and watch the multiple reflections. They fall clearly below the count recorded at a sound position. That drop shows the defect absorbs and scatters ultrasonic energy heavily. Flakes have real size, so they take energy out of the beam and the multiple reflections drop with them. The size of that drop tracks the flake class.
Take a sound area that returns five back-wall echoes. If the count falls to one, or to none at all, the flake usually rates class 4 or class 5. If it falls to two or three, the indication is usually class 3 or class 2. Clase 1 and class 0.5 flakes are the hardest to judge. Inclusions also reduce multiple reflections, so the two patterns look alike.
Reading the Position of the Defect Echoes
Flakes arise from two conditions. One is excessive hydrogen in the forging itself. The other is a process step that ran out of control in a later heat treatment. Most form during cooling after forging, when the cooling practice is wrong. Some form later, in the second heat treatment.
Why hydrogen stays in the forging
Follow cooling after forging. Hydrogen solubility in steel falls as the temperature falls. Hold the roll long enough near 400 °C and hydrogen can diffuse out. Mills call this step the hydrogen diffusion treatment. Cool too fast near 400 °C and hydrogen cannot diffuse in time. It gathers in microscopic voids, takes molecular form, and diffuses even less easily from that state. Once the stress exceeds the tensile strength of the material, that part of the steel fractures. The crack surface has not oxidised. Break it open and the fracture face is silver-white, which is where the name white spot comes from. In reality the defect is one type of crack.
Flake formation depends on the chemical composition of the steel, on its microstructure, and on the workpiece section size.
Where flakes sit in the section
On the same roll, the small-diameter journal diffuses hydrogen far better than the large section does. Como resultado, it carries few or no flakes. Flakes appear almost only in the large-diameter middle body. Inclusions and segregation run the opposite way: heavy at one end, light at the other.
Now look at the circular cross-section. Flakes sit mostly in the annular band one-third to two-thirds of the way from the centre to the surface. This holds especially for low-class flakes, class 1 a 2. Flakes from the second heat treatment belong mostly to this group and follow a ring-shaped distribution. Inclusions, porosity and segregation concentrate nearer the centre. Inside a small circle of given diameter, the defect centre and the workpiece centre do not necessarily coincide.
Finalmente, both defect families share one feature: many defect echoes appear. They differ in which zone is worst, where the defect echoes appear, and where the largest equivalent defect sits. Clase 4 and class 5 flakes carry the clearest signature. Because the defect is severe, the multiple reflections fall sharply, so operators judge them with less difficulty.
